A door, a building entrance or exit, or a device that can be opened and closed at the entrance and exit, is an entity that divides a limited space and can connect and block the entrance and exit of two or more spaces. Doors can be classified into interior doors and exterior doors. Functionally speaking, interior doors generally refer to bedroom doors and office doors, and exterior doors generally refer to security doors and fire doors.

American National Standard Institute (ANSI) ANSI 156.1 This Standard establishes requirements for butts and hinges. Cycle tests, lateral and vertical wear tests, friction tests, strength tests, finish tests, and material and dimensional requirements are included.