The substances which compose the wood cells are hydroscopic, which means they absorb moisture from a humid environment and give up moisture in dry conditions. Usually, the wood cells swell as they absorb moisture and shrink as they dry out. Because the long cells are not oriented uniformly in all directions, the amount of shrinking and swelling is also not the same in all directions. Wood is most dimensionally stable along the direction of the grain and most variable across the grain, that is, across the short width of most cells. Other properties besides dimensional stability, such as stiffness and resilience, are also greatly influenced by grain direction.
Most cracks occur along the grain, between the long wood cells.
A violin is composed of dozens of pieces of wood individually shaped and assembled with deliberately different orientations of the grain. This is done to provide maximum strength and stability, properties that are limited by the lightness of build so necessary for proper acoustical function. The sidewalls, or ribs, of the instrument must be light and thin enough to permit the vibration of the top and back plates while remaining strong enough to support the tremendous forces exerted by the taut strings. For this reason, they are cut from a straight board with the large surfaces parallel to the grain directions and are then bent to their final shape. This means that the grain direction of the ribs follows the outline of the instrument and is parallel to that of the top and back plates along the widest part of the body.