Lumber warps in many different ways, but all warp is caused by differential directional shrinkage as the wood dries from its green state. When one edge or face or end of a piece of wood shrinks more than the opposite edge or face or end, the piece warps. The three types of warp are cup, bow and crook. Following an introduction to wood shrinkage, each type of warp will be discussed In terms of its causes, likely locations in a log from which lumber might suffer from it, and preventative measures which can alleviate the warping problem, where possible.