A hoisting machine consisting usually of a vertical mast, a slanted boom, and associated tackle (see illus.). Derricks have a wide variety of forms. The mast may be no more than a base for the boom; it may be a tripod, an A-frame, a fixed column, and so on. Fixed stays may guy it in place. The boom may be fixed, it may pivot at the base of the mast, it may swing horizontally from near the top of the mast, or it may be omitted. The derrick may be permanently fixed, temporarily erected, or mobile on a cart or truck.