This article is part of our more extensive Guitar Builders’ Glossary of Terms.
4/4 (Thickness Measurement)
Your local lumber yard will describe the thickness of lumber in “quarters”. This refers to quarter-inch increments. In other words, a piece of wood 1.0” thick is 4/4 (“four quarter”), a 2.0” thick piece would be 8/4 (“eight quarter”), and so on.
Lumber can not be used immediately after a tree is harvested. The moisture content is too high and the wood will warp and twist as it dries. Air drying is the process of stacking the rough-sawn lumber with spacers between each layer to allow airflow. The moisture in the wood is released naturally, taking roughly 1 year per inch of thickness. Before air drying, the lumber is rough-sawn over-sized (thicker) so that any twisting or warping that occurs while air drying can be removed resulting in a thinner, but straight piece of wood. Air-drying usually results in a final moisture content of 15-20%.
Board Foot (bf)
Pricing pieces of wood that are random sizes is not easy for your local lumber yard. The solution is to price wood in board feet. One board foot refers to a piece of wood that is 144 cubic inches regardless of its length, width, or thickness.
An example of 1 board foot is a piece 12” wide x 12” long x 1” thick (12 x 12 x 1 = 144).
Some other examples of one board foot are:
- 24” long x 6” wide x 1” thick (24 x 6 x 1 = 144)
- 48” long x 6” wide x ½” thick (48 x 6 x ½ = 144)
Board feet is calculated as Length x Width x Thickness / 144
Here’s a real-world example. Let’s say you find a plank that’s 6 feet long (72”), 8” wide, and ¾” thick. Here’s the math:
- 72 x 8 x ¾ = 432 cubic inches
- 432 / 144 = 3bf
Book-matching refers to the process of cutting a piece of wood in half, opening the two halves like the pages of a book, and gluing them together side-by-side. The result is a piece that is twice as wide as the original, where the grain pattern of one side is the mirror reflection of the other side. Usually highly figured wood (eg. flame or quilt) is book-matched and then used as the top face of a guitar.
Burl refers to a section of a tree that develops a deformed growth resulting in a large round-ish “knob”. Burls usually contain unique, highly figured wood. They often contain unusable areas and are rarely in a uniform shape. Burls, especially maple, can be used for thin (~¼”) guitar tops.
Checking refers to cracks or stress lines usually at the ends of a board. Checks are the result of internal stresses separating the wood fibers during the drying process.
Hardness (Janka Hardness)
A test called the Janka Hardness Test is used to define the hardness of any wood. Basically, the test uses a precise method to make an indentation of a specific size and depth in the wood. The amount of force required results in a Janka Hardness Rating.
Hardwood generally comes from broad-leaf trees that lose their leaves annually. Because of this annual “slow-down”, these trees tend to be slower growing which results in hardwood lumber. Maple and Oak are examples of hardwood. The term hardwood is in no way a reference to the actual hardness of the wood [see Hardness]
Heartwood refers to the inner part of a tree (farthest from the bark) that has effectively died. It is often darker in appearance. Heartwood tends to be denser and therefore harder than outer sapwood.
Wood that has been artificially dried in a large oven as opposed to being air-dried. Kilns allow for precise control of temperature and humidity. Wood cannot be kiln-dried in a kitchen oven because there is no method to control humidity. The use of a kiln allows for reducing the moisture content to a usable 6-10% in weeks as opposed to years with air-drying.
Lineal / Linear Feet
A measurement based only on the length of a board regardless of width or thickness. For example, a board 6 feet long (72”) is 6 linear feet whether it’s 4” wide or 10”.
A mineral streak is a black, grey, or greenish streak found in some woods. Often accompanied by worm holes.
All wood contains some moisture. Moisture content is the amount of water in the wood described as a percentage of the overall weight of the wood if it were dried to have zero water content. The moisture content is determined using a moisture meter.
A pitch pocket is a cavity in wood that contains pitch, also known as resin which is the consistency of honey or thicker.
Quilt or quilting is a type of figure most often found in maple. When flatsawn, quilted maple has the appearance of rippling water.
When a tree is initially cut into oversized slabs at a sawmill, the lumber is referred to as rough-sawn.
Sapwood refers to the new-growth outer wood closest to the bark. While the inner heartwood is no longer living, the sapwood is still alive. Sapwood is generally lighter in color than heartwood.
Softwood generally comes from coniferous evergreen trees. Evergreen trees tend to grow faster making them less dense than slower-growing leaf trees (see hardwood). Spruce, Fir, and Cedar are examples of softwood.
Spalting is a color and or figure pattern caused by fungi and is mostly found in dead trees. It can appear as distinctive colors and patterns, often outlined with thin black lines.
Torification (aka Thermo treated, Roasted)
Torification is a process where wood is heated to a specific temperature range in an oxygen-free environment. The removal of oxygen is key as otherwise, the wood would simply ignite.
The process allows a number of changes that would normally happen naturally over many years to take place in a relatively short period of time. These changes include decreased moisture content, crystalization of resins, and hardening of the cell walls.
The wood becomes more stable because the cells have less ability to absorb moisture. Another notable change is that the wood darkens in appearance. Torification is not the same as kiln drying, and also can not be performed in a home oven because there is no way to remove the oxygen.
As the name suggests, these are holes (tunnels!) left in wood by burrowing worms.
Keep reading: Guitar Tonewood FAQ
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