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Components of the Body Shell
The bodies of most steel string acoustic guitars are constructed by first attaching bent, curved sides to blocks. The neck block (some builders prefer to call it a "heel block", as it supports the "heel" of their necks) is positioned at the neck end of the body. At the opposite end of the body is the tail block.
The sides then receive an additional treatment in the form of two small ledges called linings (often referred to as "kerfing", the most popular form of lining). These additions mount around the inside perimeter of both the front and the back of the sides.
Sides, blocks and linings together form a shell onto which both a braced or arched front plate (also known as a soundboard or guitar top) and a braced or arched back plate are glued, completing the body. The guitar neck, most commonly constructed independent from the body, is then attached to the body by joining it at the neck block. This is what is often referred to as a traditionally built acoustic guitar.
This article will explore the details of constructing and applying all three elements that comprise the shell of the body: the neck block, the tail block and the linings. I'd like to dedicate the article to Harold F. who inspired its writing.
My goal in presenting this information is both to inform those who are curious about how the shells or frames of acoustic guitar bodies are built as well as provide some insight into what is going on behind the scenes of issues such as tail block splits, neck block shifts and soundboard shears.
A Wood Primer
We are told that wood is composed of cellulose, hemicellulose, lignin and water. Various species have additional extractives such as oils and resins that color the wood and provide its fragrance, along with varying degrees of rot, insect and fungus resistance. Cellulose, said to be a polymer, makes up about 50% of the composition of wood and is responsible for its strength.
Lignin, an amorphous polymer, acts as a binder for the cellulose. It is often described as a matrix in which the cellulose resides, or as the adhesive that glues the wood fibers (cellulose) together. We find more lignin in the softwoods (up to one-third of the composition of the wood) than in the hardwoods, which helps to explain why you can dent Cedar with your thumbnail, and bend or break that same nail on Ebony.
The long, thin cellular structure of wood (long in the axial direction, thin in both the radial and tangential directions) is comprised of many layers of microfibrils, bundles of cellulose chains that are coated first in hemicellulose, then in lignin.
A cross section of a tree, cut perpendicular to its trunk, reveals four distinct layers:
Four Layers
The bark : the outer skinThe cambium : the actively growing layer of the treeThe sapwood : the moist, living layer of the treeThe heartwood : the inner core of the tree, comprised of darker, denser and harder material
With a new layer being added each year, the visible rings of such a cross section are (cleverly) called annual growth rings. The tree is fed via a network of tubules called xylem that supply nourishment from the roots up through the trunk, out through the branches, and into the leaves.
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How Boards are Cut
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Wood Movement
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The Neck Block
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The Tail Block
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Linings
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