Abstract: The key comprises a shank and a plurality of bits which are mounted on said shank for limited rotation relative thereto. Each bit has outwardly extending projections for engagement with the tumblers of a lock. The lock comprises a case provided with an axially extending groove. A rotor is rotatably mounted in the case and is provided with a slot. A plurality of tumblers are mounted in the rotor and each is provided with a notch. A fence member is provided which, when the notches on the tumblers are aligned with the slot, is accommodated within the notches and the rotor, and permits the rotor to be rotated relative to the case. When the notches on the tumblers are not aligned with the slot, the fence member is confined within the groove in the case and the slot, thereby preventing rotation of the rotor relative to the case. Each tumbler is provided with a central aperture to accommodate the shank of the key and is further provided with a notch to accommodate an outwardly extending projection on the key.