Checks (Checking)

Checks are small cracks that develop along the grain as wood dries, caused by the surface (or the end grain, which dries far faster than the rest of the board) losing moisture and shrinking before the interior catches up. Surface checks are usually shallow and cosmetic; end checks, which run in from a board's cut ends, can run deep enough to force cutting off several inches of otherwise good lumber.

Sealing the end grain of green or partially-dried lumber (with wax, paint, or a dedicated end-grain sealer) slows moisture loss at the ends specifically, which is exactly where checking is worst, and is standard practice for anyone air-drying their own lumber.