A door in a bulletin board system is an interface between the BBS software and an external application. The term is also used to refer to the external application, a computer program that runs outside of the main bulletin board program. Sometimes called external programs, doors are the most common way to add games, utilities, and other extensions to BBSes. Because BBSes typically depended on the telephone system, BBSes and door programs tended to be local in nature, unlike modern internet games and applications.