Glossarium (NL) 

This glossary is part of the LGRP program Phase #2. Click here to see the whole program

Defining terms used in guitar making 

Whilst definitions exist for many aspects of wood workability and physical characteristics, this tends to be in a general context and not specific to instrument (guitar) making. After making a literature survey on what is available, and obtaining input from our luthier network, we compiled this online glossary for use in guitar lutherie.


G l o s s a r y

[A] [B] [C] [D] [E] [F] [G] [H] [I] [J] [K] [L] [M] [N] [O] [P] [Q] [R] [S] [T] [U] [V] [W] [X] [Y] [Z]

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(Nl.: adhesief)

An adhesive is a substance used  to bond  two surfaces together. Examples of an adhesive substance are cement, gum, and various glues.

Annual ring

(Nl.: jaarring)

see growth zone; also 'growth ring'

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(Nl.: buigen)

is the process of placing pressure perpendicular to the longitudinal axis of the wood. A small radius produces a sharp curve and a large radius a produces a soft curve. 

Whatever method we use, be that cold, during or after steaming, by use of a bending iron or clamps we identify the following:

  • Easy to bend wood is wood which does not break and holds its shape after bending, be that a small or large radius. The wood does not twist or loose its colour permanently.
  • Care needed when bending due to specific characteristics such as: the wood discolours easily due to extraction residues ; or it returns easily to its original shape which necessitates clamping for a period of time. Extra precaution is necessary when bending to a small radius, but the wood won't break or twist anymore than other wood sorts.
  • Wood which is difficult to bend, breaks easily during bending or shows such and elasticity that it returns to its original shape even after bending. There's a possibility that the wood deforms due to internal stress, the specific grain or perhaps due to the direction the wood has been sawn.
  • Impossible to bend: Wood which breaks immediately due to its brittleness.

Bending strength

(Nl.: buigsterkte)

or ‘modulus of rupture’; or ‘maximum bending stress’; 

is the extent to which a material can bend while placed under extreme pressure on its long axis, without breaking and without extending beyond its elastic limit. The modulus of rupture is measured during a static bending test whereby the pressure is slowly increased. Just as with the modulus of elasticity the results are expressed in N/mm2.

See also: stiffnesselasticityimpact bending and strength of wood.


(Nl.: Lijmen)

Bonding is sticking materials together by means of some kind of adhesive.

We distinguish:

  • Wood which is easy to glue: the wood easily soaks up water and enough glue remains on the surface to ensure a good bonding.
  • Wood which needs preparation: The pores soak too much up and need to be sealed with extra layers of glue before final glueing (think for example about what happens if you try to glue end grain).
  • Wood which is difficult to glue: Due to the extreme density of the wood the water (glue) remains on the surface and doe not soak in. Also wood with an oily surface or covered with natural extraction residue (resins etc.) will provoke the same type of problem. It should also be noted that wood that has been sawn with a blunt blade leaves a shiny surface which also can impair bonding.


(Nl.: broosheid; versus 'taaiheid')

versus 'toughness'; Technically it’s a measurement of the ratio of stress placed upon the wood compared to the strain (deformation) that the wood exhibits along its length. Modulus of Elasticity (MOE) is expressed in pounds-force per square inch (lbf/in2) or gigapaschals (GPa). This number is given for wood that has been dried to a 12% moisture content, unless otherwise noted. (source:

Practically speaking, brittle wood splits and breaks with little effort, when twisted it easily breaks apart. It has very little flexibility. 

Sugar Maple (Acer saccharum) is a heavy wood type with its 720 kg/m3 in which we assume great strength. However this is a brittle wood. We often see wood with a large volumetric mass density are rarely flexible and depend on their strong fibres, after a heavy impact, to return to their original shape. So while sometimes very elastic they remain very stiff. Such wood types, show after heavy impact, microscopic cracks and eventually break without warning.  

Take note that while elasticity and stiffness are not contradictory to each other they are also not always comparable. Please, feel free to read more about elasticity to understand this complex notion.

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(Nl.: cambium)

The vascular cambium (also called main cambium, wood cambium, bifacial cambium; plural cambia) is a plant tissue located between the xylem and the phloem in the stem of a vascular plant. It is a cylinder of unspecialised meristem cells that divide to form secondary vascular tissues. Namely, it is the source of both secondary xylem growth inwards towards the pith and we know this as heartwood, and secondary phloem growth outwards to the bark, which we know as sapwood.  

See also: trunk structure

Colour, of heartwood

(Nl.: Kleur, van kernhout)

The colour of heartwood can vary enormously even within the same plank. The colour is influenced by various things. For example by different elements deposited within the pores, influence by UV light while growing... ,and of course there is quite often, but not always, a difference in colour between the sapwood and the heartwood. We notice less of a difference between sapwood and heartwood in trees which grow in a temperate climate than those which grow in a tropical climate. 

We differentiate:

  • Ripewood species: The more or less indistinct inner wood of some species, in which the sapwood has aged and presumably died with little if any deposition of the substances associated with heartwood. The heartwood and the sapwood of these species have the same durability. Examples of ripewood species are Beech (Fagus sylvatica) en Lime (Tilia europaea). Beech can, on occasion, show a darkening of the centre because of insufficient antibodies being produced (against bacteria and fungi) in the heartwood and is therefore only as durable as the sapwood.  
  • Sapwood species: Sapwood trees are trees with a kind of delayed lignification of the heartwood. In this the heartwood continues to function in the same way as the sapwood. This not only makes it difficult to see a colour difference between heart- and sapwood but also makes the heartwood as susceptible to threat from sickness, fungi, insect or rodent damage... as the sapwood. Examples of sapwood trees are Poplar (Populus alba) en Birch (Betula pendula).  
  • Species with obligatory colour characteristics: the wood is easily recognizable by the colour of the heartwood. Examples of such trees are African ebony (Diospyros spp.) and Purpleheart (Peltogyne spp.).

Compression wood

(Nl.: drukhout)

is reaction wood in softwood.

Curly grain

(Nl.: warrige draad)

The ordening of the cells is very irregular and runs in different directions. Curly grain occurs mostly at branch rings, trunk/root area and large forks in the tree. 
Birch (Betula pendula) is an example of European species with curly grain in the trunk. Walnut root, has this distinct pattern, and is extracted from the root of the walnut tree (Juglans spp.) Lignum-vitae (Lignum Vitae) is a tropical wood with this grain.

Cutting direction

(Nl.: zaagrichting)

Wood Cells will appear differently depending on the direction in which the timber was cut. We will consider the primary differences resulting from sawing along three different axes. Sometimes we speak of "the wood is quarter sawn, by which we mean that the growth rings are perpendicular to the the radial plane. However, some flexibility is necessary because the growth rings will always beat a variable angle to the cutting edge. Therefore, we will only speak in terms of the three axeslongitudinalradial en tangential.

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Defects due to movement

(Nl.: defecten door beweging)

The dimensional changes that accompany the shrinking and swelling of wood are major sources of both visual and structural problems in guitar making. Shrinking and swelling occur as the wood changes moisture content in response to daily as well as seasonal changes in the relative humidity of the atmosphere, i.e., when the air is humid, wood absorbs moisture and swells; when the air is dry, wood loses moisture and shrinks. When this movement occurs on different axes and at different rates, the result is deforming of the wood.  

The four most common deformations:

  • Bow (Nl.: Gebogen)
  • Crooked (Nl.: Krom) 
  • Twist (Nl.: Scheluw) 
  • Hollow (Nl.: Hol) 

See also: Forest Products Laboratory, Wood handbook, Wood as an engineering material. General Technical Report FPL-GTR-190, Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory, 2010. 

These deformations can occur or get increased because of 
the sawing direction and the direction of the growth rings (see the figure below), by the characteristics of the grain, by internal stresses, and of course during the initial phase of shrinkage when green wood dries.
Typical movements and deformations   
due to the direction of the growth rings.

These deformations can sometimes cause other defects like splitting, tearing and collapsing.


(Nl.: dichtheid; of: volumetrische massa; lb./ft3)

or 'volumetric mass density' is the unit weight expressed as lb/ft3. (or: gm/cm3; or: kg/m3.
This expresses how much one cubic foot, one cubic centimetre or one cubic meter of wood weighs.

The weight varies according to the moisture content of the wood. The density is measured when the wood has been oven dried to a moisture content of 12% or less. The density is used to form an idea of the hardness or softness of the wood.

    Remark: 'hard wood' as wood with a high density may not be confused with hardwood, which in Dutch is 'loofhout'; Softwood is in Dutch 'naaldhout'.
    The difference between hardwood and softwood says nothing about the density of the wood.
  • Lightly dense wood such as European Den (Picea abies) and White Poplar (Populus alba) are examples of wood with a density of 400 and 440 kg/m3.
  • Middle dense wood varies between 500 kg/m3 to 700 kg/m3. European Maple (Acer Pseudoplatanus L.) has a volumetric mass density of 600 kg/3m.
  • Dense wood is heavy wood and is often hard wood with a density between 700 to 1355 kg/m3. Ebony ( Diospyros Sp.Pl.) has for example a density of approximately 960 - 1120 kg/m3. Beech (Fagus sylvatica ) dried weighs 710 kg/m3.
specific gravityy    hardwood     softwood

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Early wood

(Nl.: vroeg hout; of 'zomerhout')

See: spring wood


(Nl.: elasticiteit)

or: 'Young's modulus of elasticity'

When an external pressure is exercised upon a material it returns to its original shape when that pressure is released. Almost all materials have some elasticity. By gradual implementation of a force upon a material, at some point a limit will be reached, whereby the material can no longer return to its original shape or form. This is the elastic limit of the material. Each bit of pressure beyond this limit is then called ‘permanent position’. 

Elasticity is expressed in N/mm2, and determines the stiffness of the material, and not its strength.  
Take of example a hockey stick. This could be made out of material that doesn’t break easily but has little elasticity. At the first impact the stick wouldn’t break but would bend and remain bent. This we want to avoid with hockey sticks. Therefore hockey sticks are made out of Ash (Fraxinus excelsior). Moreover ash has the capacity to absorb sudden shocks. On the other hand, as guitar builders we choose a wood that bends well and holds its new shape.  

See also: stiffness, bending strength, impact bending and wood strength.

End grain

(Nl.: kops; transversaal gezaagd hout)

A saw cut perpendicular to the grain, right through the longitudinal axis through the rings provides a front face with more or less crosscut. We choose to speak of the 'transverse plane' (X). In this area we have a nice overview of the annual rings 

See also: three axes and sawing direction.

Extraction residue

(Nl.: extractieresten)

Other than the most important substances in wood such as cellulose, hemicellulose and lignin will be known as extraction residues. Examples are resin, oil and turpentine wax. These extraction residues offer the trees a natural protection against insects and fungi. Extraction residues are common in heartwood; where they appear the wood is more dense and slightly darker. Because of these residues the wood becomes a little more stable than the sapwood. Sapwood contains almost no extraction residues and is not much resistent against insects and fungi. 

These extraction materials can have a blunting effect on cutting tools. They can also lead to difficulties in the drying of the wood making it almost impossible to impregnate.

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Fibres, of wood

(Nl.: houtvezels) 

also: cellulose fibers;

are elongated, horizontal axial cells which provide for the strength and stiffness of the plant.

is a very important polysaccharide because it is the most abundant organic compound on earth. Cellulose is a major component of tough cell walls that surround plant cells, and is what makes plant stems, leaves, and branches so strong.

Wood cells (which is more general than wood fibres) consist (mostly) of cellulose, hemicellulose and lignine.
Wood fibres consist mainly of cellulose, and contain almost no hemicellulose and lignine. 
The presence of a lot of wood fibres (such as, for example, in late wood and 95% in conifers) indicates strong  'lignification' of the timber.  

Remark: Wood fibres are not the same as 'vessels'; the latter are in fact ‘xylem’. Xylem is one of the two types of transport tissue in vascular plants, phloem being the other. The basic function of xylem is to transport water from roots to shoots and leaves, but it also transports some nutrients. The word xylem is derived from the Greek word (Xylon), meaning "wood"; the best-known xylem tissue is wood, though it is found throughout the plant. 

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(Nl.: draad) 

"grain is a ...confusingly versatile term... including the direction of the wood cells (straight grain, spiral grain), surface appearance or figure, growth-ring placement (vertical grain), plane of the cut (end grain, quarter sawn, flat sawn, etc.), rate of growth (narrow grain), relative cell size (open grain), and other meanings."

R. Bruce Hoadley              

'Grain' in the sense we'll use here is the arrangement of cels along the longitudinal axis of a tree. 

The grain type is most visible on the tangential plain
Perhaps the most important physical aspect of wood grain in woodworking is the grain direction or slope (e.g. against the grain). 
The two basic categories of grain are straight and cross grain. 

  • Straight grain runs parallel to the longitudinal axis of the piece. 
  • Cross grain deviates from the longitudinal axis in two ways, spiral grain or diagonal grain.
The amount of deviation is called the slope of the grain.

Types of grain:

In describing the application of a woodworking technique to a given piece of wood, the direction of the technique may be: 
  • with the grain (easy; giving a clean result) 
  • against the grain (heavy going; giving a poor result such as chipping or tear-out)
  • across the grain (direction of cut is across the grain lines, but the plane of the cut is still aligned with them) 
  • end grain (at right angles to the grain, for example trimming the end of a plank)

Grain, in the sense of 'Relative pore size'

(Nl.: 'nerf')

We'll use 'texture

Growth ring

(Nl.: groei ring)

also 'annual ring'.

See growth zone

Growth zone

(Nl.: groeizone)

or 'growth ring'; also 'annual ring'.

is the layers of wood produced by a single year's growth of a woody plant. In some woods these can be identified by one dark and one light ring. 

See also: 

Uit: VAN BLADEL, L., Loofhout herkennen, Praktische handleiding voor houtkenner en beginner, Sdu Uitgevers, Den Haag, 2011.

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Hard wood

(Nl.: hard hout)

different from 'hardwood';

is wood with a high density and/or specific gravity.

  • European beech (Fagus sylvatica) en ebony (Diospyros
  • Specific gravity is about 0.6-1.2
  • Density varies from 700 kg/m3 to 1355 kg/m3.
    • Note: de definition that we use here is not directly related to the Janka hardness test. This test is based on a specific interpretation of the concept of hardness whereby this indication of hardness is less useful within the musical instrument construction.


(Nl.: loofhout)

angiosperm trees; broadleaf trees

or simply 'angiospermae'; The term hardwood has nothing to do with the strength or hardness of the wood but with the cell structure.


(Nl.: kernhout)

or 'duramen'; is the innermost portion of a trunk that corresponds to the oldest growth rings formed (without the pith), and that does not contain live cells. The nutrients have disappeared from this timber and converted into other substances. Because of this, and due to the presence of extraction residues, this wood is less prone to be attacked by insects and fungi. The heartwood is generally harder, drier wood with greater density and often darker than the outer layers of wood, f.i. the sapwood. The process of cambium building up towards the pith we call 'lignification'.

See also: trunk structure

Heavy wood

(Nl.: zwaar hout)

See: specific gravity

Humidity of wood

(Nl: vochtigheidsgraad)

or 'moisture content' (MC); green, seasoned, 12%, 'after measurement')

We differentiate (here) 5 different gradations of humidity in wood: 

  • Green =.the wood is recently harvested. The cells are still full of sap. It contains both unbound and bound water. Dry wood which has become completely wet and has soaked up water is in 'green condition'.
  • Seasoned  = the wood was harvested earlier and / or cut, and the precise degree of dryness is unknown.
  • 28% = he average saturation; water is drained from the cells, but the cell walls are still filled with water. From maximum 28% humidity shrinkage starts to occur and the timber is harder.
  • 12% = the wood is air dried to it’s maximum or dried in a oven to 12% dryness or less.
  • After measurement = The wood contains …. % moisture.

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Impact bending

(Nl.: buiging onder impact; of 'weerstand tegen plotse lasten')

is the measurement of applying resistance to sudden loads; also the capacity to absorb energy.  

The impact bending is measured by subjecting a piece of wood to sudden impact by a weight being dropped on it. The height the fall determines the amount of flexibility during impact. The test of flexibility under a short impact is characterised by its short duration, and differs from for example the test to measure the bending strength of wood, in which the load gets slowly increased.

Flexibility is unimportant in guitar building unless the guitarist is planning to smash the guitar to bits after the concert.  

See also: stiffnesselasticitybending strength and strength of wood.

Interlocked grain

(Nl.: Kruisdraad)

grain which spirals around the axis of the tree, but reverses its direction for periods of years resulting in alternating directions of the spiral grain. Interlocked grain is more common in tropical wood.

The changing direction of the grain is nicely visible on the radial plain.

Although it often produces a visually stunning figure, such woods can be difficult to work because the interlocked grain tends to tear out in planing and other operations, and the wood tends to deform.

See also: grain

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Janka hardness test

(Nl.: Janka hardheidstest)

The hardness of a wood is rated on an industry wide standard known as the Janka test. The Janka test measures the force required to embed a .444 inch (11,287 mm) steel ball into the wood by half its diameter. The required force is expressed in Newton (N). This test is one of the best measures of the ability of a wood specie to withstand denting and wear.

For more information see: US DEPARTMENT OF AGRICULTURE (USDS), The Encyclopedia of Wood, Skyhorse publishing, Inc., 2007. (Kindle locatie: 2593)

Juvenile wood

(Nl.: Juveniel hout, niet te verwarren met 'jeugdhout')

The first five to fifteen rings surrounding the kern or pith of the tree are known as juvenile wood and is generally of poor quality. 

Remark: Not to be confused with sapwood. 

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Late wood

(Nl.: laat hout; of 'winterhout')

See winter wood

Light wood

(Nl.: licht hout)
  • Examples of light wood: white poplar (Populus alba and white willow (Salix alba)
  • Specific gravity around 0.15-0.45
  • Density varies from 300 kg/m3 tot 500kg/m3.
See also: specific gravity

Longitudinal axis

(Nl.: longitudinale as)

is one of the three axes in a tree trunk. The longitudinal axis runs parallel to the wood fibers, and paralelle to the central axis of a tree trunk. A cut in longitudinal direction will deliver rather quarter sawn wood. 

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Movement in wood

(Nl.: beweging, in hout)

When a tree is first felled, it is considered to be in the green state, and contains a very large amount of moisture. The humidity is reduced by drying to approximately 12%; this we call 'initial shrinking of the wood' and it means that the wood comes in a state of equilibrum with the humidity of air. Wood being hygroscopic means that it, like a sponge gains or looses moisture from the air after this initial phase of shrink. It will also expand and contract in relation to this gain or loss of moisture. In other words swelling or shrinking. This adaptation of the wood to the humidity of the surrounding air after the initial shrink we call 'movement'.  

Because the guitar builders involved in the Leonardo Guitar Research Project use mostly dried wood we shall only consider movement and the faults which are thereby caused.

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Orthotropic nature of wood

(Nl.: orthotropische aard van hout)

Wood has unique and independent of each other properties depending on the direction (on 3 axes) in which the timber is located in the trunk. Therefore  the sawing direction is a relevant parameter for the determination of the process ability (usability) of wood.

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(Nl.: merg; of 'harthout')

or 'medulla';

not to be confused with ‘heartwood’; is the middle of the tree trunk, the first growth ring. It is mostly unusable wood.

See also: trunk structure


(Nl.: schaven)

is a technique in which a strip (shaving) of material is removed, with a plane, in a cutting motion.

We differentiate:

  • Easy to plane wood: Ability to plane the wood neatly without tearing. The plane slides almost effortlessly leaving a smooth finish. The grain is straight.
  • Where extra care is needed in setting up the plane. The wood is dense and it is necessary to wet the cutting blade often. More attention should be given to setting the cutting depth of the blade. The plane tends to glide more over the surface instead of cutting, and the planing requires more effort. The grain is mostly straight. By following the above requirements one quickly achieves a smooth finish. 
  • Difficult to plane wood : the grain of the timber prevents even and easy planing. To prevent wood tear-out the wood worker has to change planing direction from time to time. 
  • Wood which is impossible to plane: The grain is as such that it is impossible to plane the wood. An alternative is to use a top router or sanding machine. An exemple of such a wood is Pink ivory (Berchemia zeyheri).

Plain sawn, or Flat sawn

(Nl.: dosse; tangentiaal gezaagd hout)

The type of wood cut is determined by the angle at which a board is cut from the log. 
There are three cuts of wood
  • Plain sawn (or: Flat sawn); 
  • Quarter sawn;
  • Rift Sawn 
Each cut produces a board of a different appearance and quality.

Plain sawn, the log is squared and sawed lengthwise in a series of parallel cuts. These cuts touch the Een zaagsnede haaks op de houtvezel, loodrecht op het radiale vlak, maar  (Lat.: 'tangere', and thus 'tangential' cut wood) growth rings. We prefer the term 'tangential plain' (T).  

The most common cut is plain sawn. The log is squared and sawed lengthwise in a series of parallel cuts. The annual growth rings appear as approximately straight lines on the board, joining at the end to form a “cathedral arch.” Because of this arch, plain sawn boards are often considered the most beautiful of the cuts. These boards are ideal for large visual areas like whole floors, tabletops, drawer fronts, sides of dressers or other similar projects. Plain sawn boards are the least expensive of the three cuts as they are the least labor-intensive to produce and leave the least waste.

See also: three axes and cutting direction.


(Nl.: porie)

a cross section of a pore is a vessel.

 If the wood is sawn perpendicular to the axis of the trunk, we see through a lens of x10 the diameter of the pores and their distribution (closed, open or far apart).  

Because all hardwood has timber vessels, we call hardwood a porous wood. Pores in hardwood, along with the grain tell us something about the texture of the wood. The texture determines the smoothness of the timber surface.

  • Ring porous wood: different angiosperms develop during the first portion of the growth increment pores with a big diameter, while during a later portion of the growth increment, they develop small pores. Examples of ring porous wood species are Ash (Fraxinus omus), Oak (Quercus spp.), Elm (Ulmus spp.), Chestnut (Castanea), Black lockust (Robinia pseudoacacia)... 
  • Diffuse porous wood: In some species (e.g. Maple (Acer spp.), Cherry (Prunus avium) and Yellow poplar (Liriodendron tulipifera) the pores are distributed fairly evenly across the earlywood and latewood. Most domestic diffuse-porous woods have relatively small-diameter pores, but some tropical woods of this type (e.g. Mahogany (Swietenia) have rather large pores. 
  • Semi diffuse porous wood: starts out growth as a ring porous wood and then becomes diffuse porous. Example: Oregon ash (Fraxinus latifolia)
  • Non-porous wood: softwoods do not have vessels they have tracheids and fiber tracheids to conduct water. These conifers can still have annual rings

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Radial axis

(Nl.: radiale as)

One of the three axes in a trunk. Eén van de  in een boomstam. The radial axis perpendicular to the wood fibres, through the growth rings in the direction of the rays. A saw cut in this direction runs across the grain. This reveals end grain.

Radial cut wood

(Nl.: kwartiers; radiaal gezaagd hout)

also: 'radial grain' or 'quarter grain';

A saw cut parallel to the wood fibre and parallel to the central axis of the trunk, in the direction of the radiant, through the heart of the tree, results in a radial plane with more or less quarter cut timber.

Both the radial axis (R) and the longitudinal axis (L) deliver this quarter cut timber. We choose "radial axis (R) rather than a quarter sawn. 

See also:  three axes and sawing direction.


(Nl.: stralen)

Rays are light coloured lines visible at the end surface (end grain). The rays run in a radial direction from the heart of the tree to the cambium. It is radially grown parenchymal tissue.

Reaction wood

(Nl.: Reactiehout) 

or 'compression wood'; or 'tension wood';

abnormally shaped wood tissue that is associated with curved stems and branches of both softwood and hardwood. It occurs when the tree takes a more natural shape under the influence of the weather (wind, rain, snow ...) or in response to other factors (light, between other vegetation and rocks ...); hence the name "reaction wood”.

  • In conifers it is generally present in the fork of the branch and in the inside bend of a curved trunk, and is called pressure timber. It is also recognisable by dark wood due to more winter growth.It can also be more opaque.
  • In deciduous trees we find this abnormal tissue in upswept branches and sometimes in a middle section. It is called tension wood. Some tree species develop more tension wood than others. The fibres feel harder and form a fuzzy surface that is difficult to work with.

Reaction wood is wood under tension. This means that, more than in other woods, deformation will take place during the drying process. More so when the lumber consists of both tension wood and normal wood. The shrink in longitudinal direction can be ten times more in reaction wood than normal wood.

Relative density

(Nl.: relatieve densiteit)

See: specific gravity

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(Nl.: Spinthout; Jeugdhout)

also called alburnum;

outer, living layers of the secondary wood of trees, which engage in transport of water and minerals to the crown of the tree. The cells therefore contain more water and lack the deposits of darkly staining chemical substances commonly found in heartwood. Sapwood is thus paler and softer than heartwood and can usually be distinguished in cross sections, as in tree stumps, although the proportions and distinctness of the two types are variable in different species. Sapwood is sensitive to fungi and is therefore not very usable unless treated chemically. As it has a high water content is is also prone to shrinkage and twisting.

See also: trunk structure


(Nl.: krimp; versus 'zwellen')

versus 'swelling'; 

when  green wood  dries, the unbound water first evaporates from the cells to a humidity of about 28%. Beyond 28% the bound water evaporates from the cell walls causing shrinkage in the wood. Mainly in a direction at right angles to the  longitudinal axis  (or at right angles to the longitudinal axis of the wood cells).

Shrinkage continues until the  moisture content  of the wood in an equilibrium comes with the humidity of a certain place and a certain time of year. In our regions this means that wood stored outdoors will reach a humidity of 12 to 15%. Conversely, the wood will swell up again with water when it is exposed to a higher than normal humidity. The wood is dried for processing in order to counteract the unwanted effects caused by shrinkage. The greatest loss of water takes place during this initial contraction phase, thereby revealing any major defects in the wood.

Wood has to be dried before processing to reduce undesired effects due to movement on the finished project. The main loss of water in wood happens during this initial phase of shrinkage, and this brings already out the worst defects. 

We consider 'shrinkage' and 'swelling' as different from 'movement in wood'.

Soft wood

(Nl.: zacht hout; niet te verwarren met 'softwood')

Soft wood is wood with a low density and / or a low specific gravity

  • Wood with a very low to low density (0-15 to 0-45) will also, almost always, have a low density (kg / m3). Here we find, among others white poplar (Populus alba 0-40 to 0-46 and European pine (Picea abies) 0-34 - 0-41 back, but also white willow (Salix alba) 0-34 to 0-40.
  • Wood with a moderate specific gravity is, for example, European cherry (Prunus avium) with 0-48 to 0-60.
  • Wood with a high to very high specific gravity, for example, European beech (Fagus sylvatica) 0.53- 0.71. European oak (Quercus pedunculata) 0.60-0.80 or African ebony (Diospyros 1.10-1.20. Deze laatste soort zinkt dan ook na droging in water.


(Nl.: coniferen; of 'gymnospermae')

or 'conifer' or 'gymnosperm' trees; 

Softwood is wood from gymnosperm trees such as conifers. The term is opposed to hardwood, which is the wood from angiosperm trees. The term ‘softwood’ has nothing to do with the strength of the tree, but it says something about the cell formation.

Specific gravity

(Nl.: specifieke gravity; ook 'relatieve densiteit')


or 'relative density', is defined as the relative density between the density of water and the density of a certain fabric (here: wood) at a temperature of 4°C and an atmospheric pressure of 1. Water has under these circumstances a density of 1000 kg/m3, or 1 gram/cm3, shortly a specific gravity of '1'. Objects with a specific gravity smaller than 1 have thus a smaller density than water, and will as a result float on water. Objects with a specific gravity bigger than 1 have a bigger density than water and will sink.

Relevance: the specific gravity of wood is generally 1,5. This means that most of the blocks of wood won't float on water. When wood dries water evaporates from the cells, the wood becomes significantly lighter and will as a result start to float on water.  Wood with a high specific gravity is heavy or dense wood, since in these woods mainly heartwood remains after the evaporation of water during the drying process. We often experience this as hard wood. Less dense wood has bigger wood fibers or sapwood which contains lots of water. After drying, less heartwood remains. The wood is light and has an open texture. We often describe it hence as light wood. A high specific gravity is a good indicator for the wood strength. We will avoid the word 'strength' because it is a container for diverse mechanical and natural characteristics who interact with each other.  

Specific gravity also is a measurement for the hardness of wood.

On this image from HOADLEY, Bruce, Understanding Wood, A Craftman's Guide to Wood Technology, The Taunton Press, Newtown, 2000, p. 14, we see a list of wood species with their specific gravity. 

As one can remark, the notions hardwood and softwood have nothing to do with hardness or strength of the wood. In other words: softwoods can be very hard and very strong.

  • Wood with very low to low specific gravity (0.15-0.45) will almost always have a low density (kg/m3). Examples of such woods are White poplar (Populus alba) 0.40-0.46 and Norway spruce (Picea abies) 0.32-0.41, but also White willow (Salix alba) 0.34-0.40.
  • Wood with a moderate specific gravity is for example European cherry (Prunus avium) 0.48-0.60.
  • Woods with high or very high specific gravity are European beech (Fagus sylvatica) 0.53-0.71, European oak (Quercus pedunculata) 0.60-0.80 or African Ebony (Diospyros 1.10-1.20. These types of wood will sink in water even after a complete drying process. 

As Eric Meier in his Wood Database comes to say:

  "In my experience, specific gravity is without a doubt the single most abused and vaguely used term in woodworking terminology."

                                                                                                Eric Meier - Wood Database, on specific gravity of wood

On his webpage he, with reason, questions the use and misuse of the notion 'specific gravity' in commercial context. It is a very interesting read that we recommend.

On this website, every effort has been made to use clear and standardised numbers for specific gravity measurements. The first number is the basic specific gravity, based on the botanical standard of oven-dry weight and green volume

The second number is meant for woodworkers, and is simply a snapshot of the wood’s specific gravity at 12% moisture content (MC), (that is, both 12% MC weight and volume). Water weighs 1,000 kilograms per cubic meter, so taking the wood’s density (in metric units) and dividing by 1,000 yields its specific gravity in woodworking standards.

Spiral grain

(Nl.: draaigroei)

The grain spirals around the axis of the tree, year after year in the same direction (to the right or to the left side). Pear (Pyrus spp.) is for instance a type of wood with spiral grain to the right. 
In general, spiral grain to the right is a sign of ageing in trees. 
Spiral grain can cause serious deformations of the wood while drying.

See also: AGENTSCHAP VOOR NATUUR EN BOS, Hout, eigenschappen en soortherkenning, Uitgave Augustus 2006, Depotnummer D/2006/3241/171, p.20.

Spring wood

(Nl.: vroeg hout ; or 'springwood')

or: early wood;

is the first shaped tissue within a growth zone. If a distinction is visible between spring and winter/summer wood,  then the diameter of the cells in spring wood are bigger than in winter woods and they often have a lighter colour. However, some species show no difference between early and late wood. ‘Winter’ and ‘spring’ wood are somewhat misleading terms as they say nothing about the season in which the wood is formed.


(Nl.: stijfheid; versus 'flexibiliteit')

versus 'flexibility'; Stiffness is the tendency of a material to maintain its original shape and size when external forces are acting upon it, and so to resist deformation. Something that is difficult to deform, is stiff. What changes form easily is flexible.

See also: elasticity and bending strength, in relation to stiffness.

Straight grain

(Nl.: rechte draad)

The cells are more or less parallel to the longitudinal axis of the tree; this is an angle of less than 45° in the longitudinal cutting plane. 

Strength of wood

(Nl.: sterkte, van hout)

is a collective term for different mechanical characteristics of wood, such as: 'toughness', 'elasticity', 'bending strength', 'impact bending', 'modulus of rupture', 'flexibility', 'compression parallel and perpendicular to the fibers' ...etc. 

We avoid the term ‘strength’ due to it’s inaccuracy.

Summer wood

(Nl.: laat hout; of ! 'winterhout')

See:  winter wood

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Tangential axis

(Nl.: tangentiale as)

One of the three three axes in a tree trunk. The tangential axis is perpendicular to the grain, and it affects the growth zones. A cut tangental to the growth rings gives plain sawn wood.

Tension wood

(Nl.: trekhout)

is reaction wood in hardwood.


(Nl.: textuur)

is the diameter of the wood vessels (xylem) and the height and width of the rays. It is the ratio of the elements of which the timber is constructed, and more specifically the arrangement of the pores. The texture is visible with a x10 magnifying glass. In some books the word ‘grain’ is used but we shall use the word ‘texture’.

  • Coarse texture: we see large pores; the wood vessels have a sizeable diameter; the rays through the wood are thick. Ex. Oak (Quercus pedunculata)
  • Medium texture: moderately large pores, medium diameter of the xylem vessels and visibility of the rays.
  • Fine texture: small or to the naked eye non-visible pores; minusculous wood vessels; almost no rays are visible. Ex. kleine tot niet-zichtbare porien; minuscule houtvaten niet met het blote oog zichtbaar; amper of geen stralen te zien. Maple (Acer spp.)

Three axes

(Nl.: drie assen)

Because of the orthotropic nature of wood the cutting direction is a relevant parameter to determine how easy the wood can be handled during different applications. 
In order to determine the (sawing) direction we distinguish three axes and one additional surface: 
  • longitudinal axis (L) = parallel to the wood fibers, and parallel to the central axis of a tree trunk 
-> a cut in this direction will deliver rather quarter sawn wood. 
  • radial axis (R) = across the growth rings, and along with the direction of the rays
 -> every cut in longitudinal direction through the inner heart of a tree will deliver (more or less) quarter sawn wood. 
  • tangential axis (T) = perpendicular to the wood fibers, and perpendicular to the radial axis, but touching the growth rings
-> a cut that strikes along the growth rings gives (more or less) plain sawn (or flat sawn) wood.
  • transversal plane (X) = perpendicular to the wood fibres, across the longitudinal axis, and across the growth rings

-> a cut across the growth rings gives (as far as possible) end grain. 



(Nl.: taaiheid; versus 'broosheid')

or 'torsion'; versus 'brittleness' or 'frailty'.

Tough wood is experienced in practice as wood that is difficult to split. Tough wood is wood that will not break until the wood is completely distorted. Tough wood is that which, even though torn, still hangs together and can be moved back and forth without breaking.Tough wood requires flexibility and has therefore the opposite of fragility.

Measurement of the toughness is done by a torsion test, and is expressed in N / mm2. The timber with tough fibres will twist after substantial and prolonged torsion and bend at right angles to the axis. On the other hand brittle wood will break off suddenly, without warning, during a test, after only a light torque. The tear out will be irregular and run obliquely with respect to the axis. 

See also: RECORD, Samuel J., The mechanical properties of wood, Forest Products Laboratory, Wisconsin,1914.

Trunk structure

(Nl.: stamopbouw).

A tree trunk exists (grossly) of the following parts named from the inside to the outside: 

pithjuvenile woodheartwoodsapwoodbast fibre and bark

When a tree grows, the cambium grows thicker to the inside under the form of heartwood and outwards under the form of sapwood.

Twisted grain

(Nl.: scheve draad)

On a radial plain straight grain can be seen. On the tangential plane we see grain that forms an angle greater than 45°.

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(Nl.: houtvat)

Vessels consist of elements which are relatively large in diameter, but have thin walls. The elements of wood vessels form long end-to-end series in the tree. At the end of their development phase the walls between the elements disappear completely. They form long tubes. Depending on the wood they can form pipes of 6 to 8 meters long. These vessels differ in diameter and distance from each other and together with the wood fibers and other elements define the texture of the wood. 
Wood vessels, if noted, are indicated in comparative terms, such as few, or numerous, rather than by precise microscopic terms such as quantity per square millimeter.

Measured on the tangential plane:
  • <25 micrometer = extremely narrow; vb. Buxus (Buxus sempervirens)
  • 25-50 mu = very narrow; vb. Willow (Salix spp.)
  • 50-100 mu = quite narrow; vb. Maple (Acer spp.)
  • 100-200 mu = average; vb. Goupie or Kabukalli (Goupia glabra)
  • 200-300 mu = quite wide; vb. Azobé (Lophira alata)
  • 300-400 mu = very wide; vb. Essessang (Ricinodendron spp.)
  • >400 mu = extremely wide; vb. Pangapanga (Millettia stuhlmannii)
Data from: VAN BLADEL, L., Loofhout herkennen, Praktische handleiding voor houtkenner en beginner, Sdu Uitgevers, Den Haag, 2011. 
Read more about this in: HOADLEY, Bruce, Understanding Wood, A Craftman's Guide to Wood Technology, The Taunton Press, Newtown, 2000, p. 18-23. 

See also: pore

Volumetric mass

(Nl.: volumieke massa)

See density expressed in kg/m3, g/cm3 or lb./ft.3

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Wavy grain

(Nl.: golvende draad)

Characteristic of wavy grain is that it does not run parallel with the core of the tree. A wavy surface will result from splitting this wood tangentially. Finishing this wood will deliver a stripy effect (Moiré effect) which is sought after by luthiers. A good example of this is maple (Acer pseudoplatanus L.).

Winter wood

(Nl.: laat hout; of 'winterhout'; ! Eng.: 'summerwood')

of 'laat hout'; 

winter wood is a sections of a growth zone. Winter woods as an average are denser and darker than spring wood. However, some species exhibit little difference between early and late wood. ‘Winter’ and ‘spring’ wood are somewhat misleading terms as they say nothing about the season in which the wood is formed.

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