Wood Species Library

Wood Glossary

Key terms and definitions for understanding wood species data, properties, and classifications.

88 terms

A

Acclimation: The process of allowing lumber to reach equilibrium moisture content with its environment before installation. Critical for flooring and cabinetry to prevent post-installation warping.

Air-Dried: Lumber dried naturally outdoors in stacked, stickered piles. Typically reaches 12-20% moisture content depending on climate. Slower but less stressful to wood than kiln drying.

B

Blunting Effect: The tendency of a wood species to dull cutting edges. High silica content or interlocked grain increases blunting. Rated low, moderate, high, or severe.

Board Foot: A unit of volume for lumber equal to 144 cubic inches (1 ft × 1 ft × 1 in). Used to measure and price hardwood lumber.

Book Match: A veneer or board arrangement where consecutive sheets are flipped like pages of a book, creating a mirrored grain pattern. Common in fine furniture and panel work.

Bow: A warp along the length of a board where the face curves away from flatness end-to-end. Caused by uneven drying or internal stress.

Burl: An abnormal growth on a tree that produces highly figured, swirled grain patterns. Burl wood is prized for decorative veneers and turned objects.

C

Cambium: The thin layer of living cells between the bark and sapwood that produces new wood and bark cells. The growth engine of a tree.

Case Hardening: A drying defect where the outer shell of a board dries and sets while the interior remains wet. Creates internal stress that can cause cracking when the board is resawn.

Chatoyancy: A shimmering, shifting light effect seen in some woods (like Sapele ribbon stripe or curly Maple). Caused by fibers at alternating angles reflecting light differently.

Check: A crack that develops along the grain, typically at the end of a board, caused by uneven drying. End-grain sealer helps prevent checks during drying.

CITES: Convention on International Trade in Endangered Species. An international treaty regulating trade in threatened species. Appendix I = most restricted; Appendix II = regulated; Appendix III = monitored.

Clear Grade: Lumber free of knots, splits, and other defects. The highest visual grade, used for fine furniture, trim, and visible surfaces.

Cross Grain: Wood fibers that deviate significantly from the longitudinal axis of the board. Reduces strength and can cause finishing problems.

Crushing Strength: The maximum compressive stress a wood can withstand parallel to the grain before failure. Measured in psi. Important for structural applications.

Cup: A warp across the width of a board where the face curves. Flatsawn boards are more prone to cupping than quartersawn. Caused by differential shrinkage.

Curly Figure: A wavy, rippled pattern in wood grain that creates a three-dimensional optical effect. Also called 'tiger stripe' or 'fiddleback.' Common in Maple and Koa.

D

Decay Resistance: A wood's natural ability to resist rot from fungi. Rated from 'Perishable' (least resistant) to 'Very Durable' (most resistant). Heartwood is typically more resistant than sapwood.

Defect: Any irregularity in wood that reduces its strength, durability, or aesthetic value. Includes knots, splits, wane, warp, stain, and decay.

Dimensional Lumber: Softwood lumber cut to standardized widths and thicknesses (e.g., 2×4, 2×6). Nominal dimensions differ from actual: a 2×4 actually measures 1.5" × 3.5".

E

Earlywood: The lighter, less dense portion of an annual growth ring formed during the fast-growth spring season. Wider earlywood bands indicate faster growth.

Elastic Modulus (MOE): Modulus of Elasticity — measures a wood's stiffness or resistance to bending. Measured in Mpsi (million psi). Higher = stiffer.

End Grain: The cross-section of wood when cut perpendicular to the grain direction. End grain patterns are key for species identification.

Equilibrium Moisture Content (EMC): The moisture content at which wood stops gaining or losing moisture to its environment. Typically 6-8% indoors and 12-14% outdoors in most climates.

Extractives: Chemical compounds deposited in heartwood that give it color, odor, decay resistance, and other properties. Include tannins, oils, resins, and pigments.

F

Face Grain: The wide surface of a board showing the typical wood grain pattern. This is the surface seen in most finished applications.

FAS (Firsts and Seconds): The highest grade of hardwood lumber under NHLA rules. Boards must be at least 6" wide, 8' long, and yield 83.3%+ clear cuttings.

Fiber Saturation Point (FSP): The moisture content (typically ~28-30%) at which cell walls are fully saturated but cell cavities are empty. Below FSP, wood begins to shrink.

Fiddleback: A type of curly figure with tight, regular waves perpendicular to the grain. Named for its traditional use in violin backs. Prized in Maple, Sycamore, and some tropical species.

Figure: Distinctive visual patterns in wood beyond basic grain, such as quilting, curly, birdseye, or ribbon stripe. Caused by growth irregularities or grain angle variations.

Flat Sawn: The most common sawing method where boards are cut tangent to growth rings. Produces cathedral-pattern grain. Also called plain sawn. More prone to cupping than quartersawn.

G

Grain: The orientation and pattern of wood fibers. Common types: straight (parallel fibers), interlocked (alternating spiral), wavy (undulating), and irregular.

Green Wood: Freshly cut lumber that hasn't been dried. Contains high moisture content and will shrink as it dries. Not suitable for most finished applications until kiln-dried or air-dried.

Growth Ring: An annual ring visible in cross-section, formed by the contrast between earlywood (spring) and latewood (summer). Ring width indicates growth rate.

H

Hardwood: Wood from deciduous (broadleaf) trees, regardless of actual hardness. Classified by botanical structure, not density. Some hardwoods (e.g., Balsa) are softer than most softwoods.

Heartwood: The dense, often darker inner wood of a tree trunk. No longer actively transporting sap. Usually more durable and decay-resistant than sapwood.

Honeycombing: An internal drying defect where checks develop inside the board while the surface appears sound. Caused by case hardening followed by the interior shrinking.

I

Interlocked Grain: Grain where fibers spiral in alternating directions between growth layers. Creates a ribbon-stripe figure on quartersawn surfaces. Can cause tearout when machining.

IUCN Status: Conservation status assigned by the International Union for Conservation of Nature. Ranges from 'Least Concern' to 'Critically Endangered'. Indicates the species' risk of extinction in the wild.

J

Janka Hardness: The standard measure of a wood's resistance to denting and wear. Measured in lbf (pounds-force). The test measures force required to embed an 11.28mm steel ball halfway into the wood. Key metric for flooring and high-traffic applications.

K

Kerf: The width of the cut made by a saw blade. A wider kerf wastes more material. Thin-kerf blades maximize yield from expensive hardwoods.

Kiln-Dried (KD): Lumber dried in a controlled oven (kiln) to reduce moisture content, typically to 6-8% for interior use. Produces more stable, predictable lumber than air-drying.

Knot: The cross-section of a branch embedded in lumber. Tight knots are sound and fixed; loose knots can fall out. Knots reduce structural strength and can complicate finishing.

L

Lacey Act: A U.S. law (amended 2008) that prohibits trade in illegally sourced wood products. Requires due diligence on supply chain legality for imported lumber.

Latewood: The denser, darker portion of an annual growth ring formed during slower summer growth. Higher latewood proportion generally means harder, stronger wood.

Linear Foot: A measurement of length only (1 foot = 12 inches), regardless of width or thickness. Used for trim, molding, and some dimensional lumber pricing.

Live Edge: A board that retains the natural, irregular edge of the tree, including bark or cambium. Popular for tables and decorative pieces that showcase the organic shape.

Luster: The way wood reflects light. Ranges from low (matte) to high (glossy/silky). Affected by cell structure and extractives.

M

MBF: Thousand board feet. Standard unit for wholesale lumber trading and mill production. 1 MBF = 1,000 board feet.

Medullary Rays: Ribbons of cells radiating from the center of a tree outward. Visible as ray fleck in quartersawn Oak, Beech, and Sycamore. Important for species identification.

Modulus of Rupture (MOR): The maximum bending stress a wood can withstand before breaking. Measured in psi. Higher values indicate stronger wood in bending applications.

Moisture Content (MC): The percentage of water in wood relative to the oven-dry weight. Fresh-cut wood is 30-200% MC; kiln-dried is 6-8%; air-dried is 12-20%.

Moisture Meter: A device that measures wood moisture content. Pin-type meters measure electrical resistance between two probes. Pinless meters use electromagnetic signals.

Monocot: A plant classification that includes bamboo, palms, and rattan. Not true wood in the botanical sense but commercially used as wood alternatives.

N

Nominal Size: The name size of lumber before milling and drying. A '2×4' is nominally 2" × 4" but actually 1.5" × 3.5" after surfacing.

O

Old Growth: Wood harvested from ancient, undisturbed forests. Characterized by tight growth rings, high density, and superior stability compared to plantation-grown wood.

Open Grain: Wood with large, visible pores (vessels), like Oak or Ash. Requires grain filler for a perfectly smooth finish. Also called ring-porous.

Outdoor Suitability: A species' ability to perform in exterior applications. Depends on decay resistance, stability, and weathering characteristics. Rated poor, fair, good, or excellent.

P

Patina: The natural color change wood undergoes with exposure to light and air over time. Cherry darkens dramatically; Walnut lightens; Ipe silvers.

Pith: The soft, spongy center of a tree trunk. Boards containing pith are structurally weaker and more prone to splitting.

Q

Quartersawn: Lumber cut with growth rings roughly perpendicular to the board face (60-90°). Produces straighter grain, better stability, and can reveal ray fleck patterns in species like Oak.

R

Reaction Wood: Abnormal wood formed when a tree grows at an angle. Compression wood in conifers; tension wood in hardwoods. Both are unstable and prone to unpredictable movement.

Resin: Natural organic compounds found in softwoods (pine pitch) and some hardwoods. Can interfere with finishing and gluing. Some resins provide natural decay resistance.

Rift Sawn: Lumber cut at 30-60° to growth rings, between flat and quartersawn. Produces consistent, straight-line grain without cathedral patterns or ray fleck.

Ring-Porous: A hardwood with distinctly larger pores in earlywood than latewood, creating a visible ring pattern (e.g., Oak, Ash, Elm). Contrast with diffuse-porous woods like Maple.

Rough Sawn: Lumber as it comes off the saw, before planing or surfacing. Thicker and rougher than finished lumber. Sold at nominal dimensions.

S

S4S: Surfaced Four Sides — lumber that has been planed smooth on all four faces and edges. The most common retail preparation.

Sapwood: The outer, usually lighter-colored wood of a living tree. Actively transports water and nutrients. Generally less durable than heartwood.

Shrinkage (Radial): The amount wood shrinks across the radius of the tree (from center outward). Measured as a percentage from green to oven-dry.

Shrinkage (Tangential): The amount wood shrinks along the growth rings (tangent to rings). Usually about twice radial shrinkage. Main cause of warping.

Shrinkage (Volumetric): Total volume change from green to oven-dry state. Combines radial, tangential, and longitudinal shrinkage.

Silica: Mineral deposits found in some tropical hardwoods (Teak, Ipe). Increases wear resistance but rapidly dulls cutting tools. Measured as silica content percentage.

Snipe: A shallow cut at the beginning or end of a board caused by a planer or jointer lifting the wood slightly. Minimized by supporting boards and using sacrificial pieces.

Softwood: Wood from coniferous (needle-bearing) trees like pine, spruce, and fir. Classified by botanical structure. Some softwoods (e.g., Yew) are actually quite hard.

Spalting: Distinctive dark zone lines created by competing fungi in partially decayed wood. Prized for decorative work but indicates reduced structural integrity.

Specific Gravity: The ratio of a wood's density to water's density. A key predictor of strength properties. Higher specific gravity generally means harder, stronger, heavier wood.

Stability Rating: How much a wood moves (expands/contracts) with moisture changes after drying. Rated from 'Low' (moves a lot) to 'Very High' (very stable). Critical for flooring and precision work.

Sticker: A thin strip of wood placed between layers of stacked lumber to allow air circulation during drying. Sticker stain occurs when moisture gets trapped under the strip.

Surfaced: Lumber that has been run through a planer to create smooth, parallel faces. S2S = surfaced two sides; S4S = surfaced four sides.

T

T/R Ratio: Tangential-to-Radial shrinkage ratio. Indicates how uniformly wood shrinks. Closer to 1.0 = more uniform shrinkage = less tendency to warp. Most woods range 1.5-2.5.

Tannin: Natural chemical compounds in wood (especially Oak) that react with iron and water, causing dark staining. Also affects glue bonding and finish adhesion.

Tearout: Fibers ripped from the surface during machining, leaving a rough, torn patch. Common with interlocked or figured grain. Minimized by sharp tools and shallow cuts.

Texture: The relative size and variation of wood cells. Fine texture (small, uniform cells) produces smoother surfaces. Coarse texture (large cells) creates a more open-pored surface.

Twist: A warp where the four corners of a board do not lie in the same plane. One of the most difficult warps to correct. Caused by spiral grain or uneven drying.

V

Veneer: A thin slice of wood (typically 1/42" to 1/8" thick) peeled or sliced from a log. Used for decorative faces on plywood, furniture, and architectural panels.

W

Wane: The presence of bark or missing wood along the edge of a board. Common in lower lumber grades. Some designs intentionally incorporate wane for a rustic look.

Warp: Any distortion from flatness in a board. Includes bow, cup, twist, and crook. Caused by uneven drying, internal stress, or grain irregularities.

Workability: How easily a wood can be machined, sanded, glued, and finished. Affected by grain pattern, density, silica content, and resin/oil content. Rated easy, moderate, difficult, or very difficult.