Defines the standard interface dimensions for the type FS connector family which is a duplex connector set of plug and adaptor configuration. The plug has a pair of cylindrical spring loaded abutting ferrules of 2,5 mm nominal ferrule diameter. The optical alignment mechanism is a rigid bore or resilient sleeve contained within the adaptor. It includes a hand-released latch coupling. The plug has multiple keyway arrangements and the adaptor has multiple key configurations. The keying scheme is exclusionary and is used to limit mating between plug and adaptor to specific key combinations.