A key system or key telephone system is a multiline telephone system typically used in small office environments.
Key systems are noted for their expandability and having individual line selection buttons for each connected phone line, however some features of a private branch exchange such as dialable intercoms may also commonly be present.
Key systems can be built using three principal architectures:
Electromechanical shared-control
Electronic shared-control
Independent keysets
Reference:
http://en.wikipedia.org/wiki/Key_telephone_system
