Cup: the board remains flat in length and on edge but curves across its width, away from the heart.

Bow: the board remains flat in width and on edge but curves in length, like a straight road descending a hill.

Spring: the board remains flat in width but curves in length, like a river going around a bend.

Twist: the board curves in length and width like a propeller.

A board may be distorted in one of four ways: cupped, sprung, bowed or twisted.

Cupping is almost always associated with flatsawn cathedral figure board. Look at the end grain and you will see which side is nearer the center of the tree. The board has shrunk more on the surface away from the heart, causing it to curl, as though the growth rings were trying to straighten out. If you look at the end grain of a quartersawn board, growth rings are mostly perpendicular to the surface of the board. The tangential shrinkage is the same on both sides, with the result that the board stays flat.

A sprung board is flat but bent, like a river going around a corner. The bend is normally too minor to prevent harvesting useful parts. A bowed board is also flat, but bent, like a road going over a hill. A twisted board is shaped like a propeller. Severely twisted boards are seldom useful.

These distortions are frequently the result of an abnormal tree trunk or poor kiln drying. Whatever the cause, they are amplified and easily seen in a board that is six to eight feet long and nine inches wide. Such a board should not be rejected on the grounds of distortion alone, because when it is broken down to furniture-sized parts, the distortions are much diminished. Often, one pass over the jointer and a trip through the planer takes care of it.

Wood shrinks and expands in response to water vapor in the air — the humidity. It takes on water vapor and expands when the humidity is high. It gives back water vapor and shrinks when the air is drier. All of this shrinkage and expansion is across the grain. It remains constant in length. A material that behaves this way is said to be hygroscopic. What it does is try to get totally in sync with the moisture content of the surrounding air. We call it moisture equilibrium. In reality, it doesn't happen because water vapor in the air changes daily and, more important, seasonally. It's generally more humid in summer and less humid in winter, and wood reacts to this seasonal change by shrinking and expanding. That's why doors and drawers that move freely in winter tend to stick in summer. It's a slow process, so the day-to-day changes have little effect. However, it is this dimensional change that traditional furniture construction techniques are designed to accommodate, frame-and-panel being the best example. The narrow frame members are made rigid while the wide panel sits loosely in a groove so that it can freely shrink and expand within the rigid frame.

But there is a complication. Wood doesn't shrink and expand the same amount in all directions. When it is being dried, it shrinks almost twice as much one way as it does the other. The two directions of shrinkage are based on a circle because a section of the trunk is a circle as are most of the elements that make up wood, so shrinkage is almost twice as great in the tangential direction as in the radial direction. We call this quirk of nature shrinkage differential, and it's the sole reason why wood distorts in the four ways already described.