The 2.14 mm FC/APC key format is typically called "wide key", "NTT" or "type N". The narrower 2 mm key format is called "Reduced" or "type R". Type R plugs will mate to any receptacle, but will fail to maintain precise rotation angle unless the receptacle is also type R. Type N plugs will not mate with type R receptacles but will mate with FC/PC receptacles. FC/PC plugs will mate with type N receptacles and may or may not mate with type R receptacles. Some manufacturers mark type R keys with a single scribe mark on the key, and type N keys with a double scribe mark.
