A keystone jack is a female connector used in data communications, particularly local area networks (LANs). The jack is usually mounted in a wall plate or patch panel. A keystone plug is the matching male connector, usually attached to the end of a cable or cord.
A principal advantage of keystone connectors is their versatility. Several types of keystone jack can be mounted on a single patch panel. They are available in unshielded and shielded forms, and can accommodate cords and cables having various numbers of conductors.
The term keystone derives from the characteristic shape of the jack, resembling the standard RJ-11 wall jack used to connect telephone sets, fax machines, and dial-up computer modems to conventional telephone lines.
Dig Deeper on LANs (Local Area Networks)