A monkey wrench is an old type of wrench with an adjustable jaw. In contrast to a pipe wrench, which has teeth and a loosly mounted head that allows it to bite into a pipe and tighten as a strain is put on the handle, a monkey wrench has adjustable, smooth jaws that always stay parallel. The monkey wrench is used for turning pipe unions, the heads of bolts, and other flat surfaced objects. For automotive work, the monkey wrench has been largely superceded by the cresent wrench and other more modern adjustable wrenches.