An interlock is a feature that makes the state of two mechanisms or functions mutually dependent. It may be used to prevent undesired states in a finite-state machine, and may consist of any electrical, or mechanical devices or systems. In most applications, an interlock is used to help prevent any damage to the machine or to the operator handling the machine. Elevators are equipped with an interlock that prevents the moving elevator from opening its doors, and prevents the stationary elevator (with open doors) from moving. Interlocks may include sophisticated elements such as curtains of infrared beams, photodetectors, simple switches, and locks. It can also be a computer containing an interlocking computer program with digital or analogue electronics.