Crouching is not present in vanilla Doom and the other classic games, however ZDoom added this feature. Crouching can be selectively enabled or disabled through MAPINFO map definitions and through the gameplay options menu. The console variables sv_allowcrouch and sv_nocrouch govern whether crouching is possible, possible if not disallowed by the map, or impossible.
If a player class is not provided crouch sprites with the Player.CrouchSprite, the ZDoom uses the normal sprite vertically shrunk. This is not an ideal solution, but it has the merit of actually showing the player is crouching. A crouched player's height is equal to half its normal height. Monsters cannot crouch.