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These are terms you’ll use throughout your woodworking program.

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A
Air-dried lumber
Boards that have dried naturally by stacking them in the open air, as shown above and right. Air flows between the boards, allowing the moisture in the wood to evaporate. Air drying can take as long as one year per inch of board thickness. In all but the driest regions, moisture content rarely falls below 12 to 20 percent without additional drying indoors.
Angiosperms (Angiospermae)
Botanical name for all plants whose seeds are carried inside an ovary, including what are commonly referred to as hardwoods: those trees which are broad-leaved, flowing and fruit-bearing. Most are deciduous.
Annual or growth rings
Concentric rings of wood added yearly to the growing tree in temperate zones.
B
Backsawn
Sawn so that the growth rings are inclined at less than 45 degrees to the wide face. A backsawn log is converted in such a way as to provide the maximum number of cuts tangential to the growth rings.
Bast
The soft, fibrous tissue between the bark and the inner cell structure.
Board foot
The standard unit of measure for hardwood lumber. Because hardwoods often sell in random widths and lengths, a board foot measures thickness, width, and length to determine the total volume of wood in the board. Sometimes called a “Board Root.”
Bole
The part of the trunk or stem of the tree from above the root butt to the first branch or limb, normally of timber size over 200mm in diameter.
Bow
A warp in which the ends of a board or wooden member curve in the same direction away from the desired plane, usually along the length.
Burl
A swirling, twisted figure in wood grain caused by growths on the outside of the tree or root.
C
Cambium
The layer of cells in a tree that divides to produce new tissue.
Checks
Longitudinal separation of the fibers in wood that do not go through the whole cross section. Checks result from tension stresses during the drying process.
Close-grained
Having narrow growth rings.
Compressive Strength Parallel to Grain
Maximum stress sustained by a compression parallel-to-grain specimen having a ratio of length to least dimension of less than 11.
Compressive Stress Perpendicular to Grain
Reported as stress at proportional limit. There is no clearly defined ultimate stress for this property.
Crown
The uppermost branches and foliage of a tree.
Crown-sawn
A less common term for flat-sawn.
D
Deciduous
In the fields of horticulture and botany, the term deciduous means “falling off at maturity” and “tending to fall off”, in reference to trees and shrubs that seasonally shed leaves, usually in the autumn; to the shedding of petals, after flowering; and to the shedding of ripe fruit.
Density
Weight per unit volume. Density of wood is influenced by rate of growth, percentage of late wood and in individual pieces, the proportion of the heartwood.
Diffuse-porous
(of a hardwood) having pores that are typically of uniform size throughout the growth ring.
Dimensional Stability
A term that describes whether a section of wood will resist changes in volume with variation in moisture content (other term: movement in performance).
E
End grain
The grain shown on a crosscut surface, revealing the cut ends of the wood fibers.
F
Fiber Saturation Point
Fiber saturation point is a term used in wood mechanics and especially wood drying, to denote the point in the drying process at which only water bound in the cell walls remains – all other water, called free water, having been removed from the cell cavities.
Figure
The pattern produced in a wood surface by annual growth rings, rays, knots, deviations from regular grain, such as interlocked and wavy, and irregular coloration.
G
Gnetophytes
Gnetophyta is a division of plants, grouped within the gymnosperms, that consists of some 70 species across the three relict genera: Gnetum, Welwitschia, and Ephedra. Fossilized pollen attributed to a close relative of Ephedra has been dated as far back as the Early Cretaceous.
Grain
The direction, size, arrangement, appearance, or quality of the fibers in sawn wood. Straight grain is used to describe lumber where the fibers and other longitudinal elements run parallel to the axis of the piece.
Grain
The arrangement of the fibres that make up the wood, or the pattern produces by these fibres on the surface of the wood. Many types of grain pattern are distinguished, such as fine, coarse, plain, interlocked, etc. The word grain tends to refer to the pattern of the wood, whereas figure refers to interesting irregularities. Timber species can be identified by their own unique, distinctive grain patterns.
Grain direction
The direction in which the dominating, elongated fibers or cells lie in the structure of wood.
Green wood
Stock, usually in rough-cut lumber or log form, that has been cut but not dried, and retains a high moisture content. Woodturners often use green stock because of its workability.
Gum Pocket
An excessive local accumulation of resin or gum in the wood.
Gymnosperm
The gymnosperms, also known as Acrogymnospermae, are a group of seed-producing plants that includes conifers, cycads, Ginkgo, and gnetophytes. The term “gymnosperm” comes from the composite word in Greek: γυμνόσπερμος, literally meaning “naked seeds”. The name is based on the unenclosed condition of their seeds.
Gymnosperms (Gymnospermae)
The conifers and related plants, whose seeds are not contained without an ovary; they have needle-like leaves, and produce the softwoods.
H
Hardness
Generally defined as resistance to indentation using a modified Janka hardness test, measured by the load required to embed a 11.28 mm (0.444 in.) ball to one-half its diameter. Values presented are the average of radial and tangential penetrations.
Hardwood
Wood derived from broadleaf trees—oak, walnut, ash, and cherry, for example. In temperate regions, these trees are deciduous, dropping their leaves annually. Called angiosperms, the trees produce seeds in the form of fruits or nuts. Not all hardwoods are hard and heavy. Balsa, for example, is classified as a hardwood although it contains light, soft wood.
Heartwood
The inner layers of wood in growing trees that have ceased to contain living cells. Heartwood is generally darker than sapwood, but the two are not always clearly differentiated.
Hygroscopy
Hygroscopy is the phenomenon of attracting and holding water molecules via either absorption or adsorption from the surrounding environment, which is usually at normal or room temperature.
I
Impact Bending
In the impact bending test, a hammer of given weight is dropped upon a beam from successively increased heights until rupture occurs or the beam deflects 152 mm (6 in.) or more. The height of the maximum drop, or the drop that causes failure, is a comparative value that represents the ability of wood to absorb shocks that cause stress beyond the proportional limit.
Interlocked Grain
Grain that exhibits periodic changes in the direction and pitch of the fibres, often producing a ribbon-like figure.
J
Janka Hardness Test
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 steel ball into the wood by half its diameter.  This test is one of the best measures of the ability of a wood species to withstand denting and wear. It is also a good indicator of how hard a species is to saw, mill and nail.
K
L
Lumber
Lumber, also known as timber, is a type of wood that has been processed into beams and planks, a stage in the process of wood production. Lumber is mainly used for structural purposes but has many other uses as well. 
M
Medullary rays
Vertical ribbons or sheets of tissue formed radially across the annual rings, which are very distinctive in some woods, such as oak (Quercus spp.), and barely visible in others
Modulus of Elasticity
An imaginary stress necessary to stretch a piece of material to twice its length or compress it to half its length. Values for the individual species are given in megapascals (MPa – equivalent to N/m2), and are based on testing small clear pieces of dry wood.
Modulus of Rupture
Reflects the maximum load-carrying capacity of a member in bending, and is proportional to maximum moment borne by the specimen. Modulus of rupture is an accepted criterion of strength, although it is not a true stress because the formula by which it is computed is valid only to the elastic limit.
Moisture content (MC)
The moisture content of wood decreases dramatically during seasoning. The completely dry (oven-dry or kiln dried) weight of a given species is a constant, and the moisture content of the wood at any given time can be express as a percentage of this constant. The content can be determined using a moisture meter. For kiln-dried stock, moisture content generally runs from 4 to 10 percent.
The formula: MC = weight of water present in sample / oven-dry weight of sample * 100
N
O
P
Phloem
The inner bark, used for food transport in the growing tree
Pith
The soft core in the center of a tree trunk, branch or twig.
Pith Flecks
Pith-like irregular discolored streaks of tissue in wood, due to insect attack on the growing tree.
Plain-Sawn
Plain-sawn (plainsawn) hardwood boards are produced by cutting tangentially to a tree’s growth rings, creating the familiar “flame-shaped” or “cathedral” pattern. This method also produces the most lumber from each log, making plain-sawn lumber a cost effective design choice. Same meaning as “flatsawn.”
Q
Quarter-sawn
Quarter-sawing means cutting a log radially (90-degree angle) to the growth rings to produce a “vertical” and uniform pattern grain. This method yields fewer and narrower boards per log than plain sawing, boosting their cost significantly. Quarter-sawn boards are popular for decorative applications such as cabinet faces or wainscoting. They will expand and contract less than boards sawn by other methods.
R
Rift-sawn
Rift-sawing at a 30-degree or greater angle to the growth rings produces narrow boards with accentuated vertical or “straight” grain patterns. Rift-sawn boards are often favored for fine furniture and other applications where matching grain is important. This type of lumber is available in limited quantities and species.
Rough-sawn
Boards—typically hardwoods—cut to thickness, and sometimes width, during the initial milling process. This leaves telltale rough, splintery surfaces on all sides. Does not include planning or re-ripping.
S
S2S
A lumber-industry abbreviation for “surfaced on two sides”. These boards are planed on both faces to final thickness after milling and drying. Typical S2S Thicknesses (hardwoods):
Nominal — Actual
4/4 (1″) —1316 “
5/4 (114 “) — 1116
6/4 (112 “) — 1516 “
8/4 (2″) — 134 “
S3S
An abbreviation for “surfaced on three sides”. Here, boards get planed on both faces, and then straight-line ripped on one edge, shown below. Most hardwood sells as S3S or S2S.
S4S
An abbreviation for “surfaced on four sides”. These boards get planed on both faces, and then ripped on both edges to make them parallel, shown above. Most often, this process produces “dimensional” lumber in standard sizes, such as 1×6, 2×4, and so forth. You’ll find softwood construction lumber sold this way, as well as hardwoods in home centers.
Sapwood
The outer zone of wood in a tree, next to the bark. Sapwood is generally lighter than heartwood.
Shear Strength Parallel to Grain
Ability to resist internal slipping of one part upon another along the grain. Values presented are average strength in radial and tangential shear planes.
Shrinkage
The contraction of wood fibers caused by drying below the fiber saturation point (usually around 25-27% M.C.). Values are expressed as a percentage of the dimension of the wood when green.
Softwood
Wood derived from needleleaf trees—spruce, pine, fir, and cedar, for example. Commonly known as conifers (gymnosperm), these trees produce seeds encased in cones, and are also called gymnosperms. Softwood trees are almost always evergreen, retaining their needles year-round. Some softwoods, such as spruce, are soft, but others, such as ponderosa pine, are hard and remarkably strong.
Species
A subdivision of genus; a group of individual plants of the same kind that share many of the same characteristics. The genus Quercus encompasses all types of oak, whereas Quercus robur is the name of one particular species of European oak.
Split
Separation of the fibers in a piece of wood from face to face (other term: end-split).
T
Tensile Strength Perpendicular to Grain
Resistance of wood to forces acting across the grain that tend to split a member. Values presented are the average of radial and tangential observations.
Texture
Determined by relative size and distribution of the wood elements. Described as coarse (large elements), fine (small elements) or even (uniform size of elements).
Tracheids
Tracheids are elongated cells in the xylem of vascular plants that serve in the transport of water and mineral salts, and fond in softwoods. Tracheids are one of two types of tracheary elements, vessel elements being the other. Tracheids, unlike vessel elements, do not have perforation plates.
U
V
Vessel Elements
A vessel element or vessel member is one of the cell types found in xylem, the water conducting tissue of plants. Vessel elements are typically found in angiosperms but absent from most gymnosperms such as conifers.
W
Warp
Distortion in lumber causing departure from its original plane, usually developed during drying. Warp includes cup, bow, crook and twist.
Weight
The weight of dry wood depends upon the cellular space, the proportion of wood substance to air space.
Woolly grain
A woolly or fuzzy surface with frayed, rather than cleanly cut fibres, after machining. It can occur with tension wood, or can be a feature of certain species.
Work to Maximum Load in Bending
Ability to absorb shock with some permanent deformation and more or less injury to a specimen. Work to maximum load is a measure of the combined strength and toughness of wood under bending stresses.
X
Xylem
The living tissue in the outer layers of the tree trunk, serving to transport sap and store food; known as sapwood when converted to timber.
Y
Z