A keying system for providing selective interconnection between a receptacle socket and a connector plug terminating an optic fibre, the connector plug adapted for insertion into the receptacle socket. The keying system comprises a raised boss configured to one of a plurality of predefined boss keying geometries and a cavity configured to one of a plurality of predefined cavity keying geometries. The boss is either in one of the receptacle socket or on a forward end of the connector plug and the cavity is formed in the other of the receptacle socket or the forward end of the connector plug. At least one of the predefined boss keying geometries matches at least one of the predefined cavity keying geometries. When the boss keying geometry matches the cavity keying geometry, the boss can be inserted into the cavity thereby interconnecting the connector plug with the receptacle socket. In this manner correct interconnections between connectors and other connectors or sockets can be insured thereby providing an enhanced level of security.