Dual control lock

Claim 1

A lock apparatus comprising: housing means defining, on an inner surface thereof, primary spline means and secondary spline means substantially parallel thereto and further defining trapping spline means; plug means rotatably retained in said housing, said plug means defining a keyway therethrough; a primary tumbler for releasably restraining said plug means in an unlocked position with said primary tumbler interacting with said primary spline means, and for releasably restraining said plug means in a locked position with said primary tumbler interacting with said secondary spline means and wherein said primary tumbler is releasable by a primary key to allow rotation of said plug from said unlocked to said locked position; and a trapping tumbler non-responsive to the primary key and engageable with said trapping spline means to releasably trap said plug means in the locked position so as to prevent rotation thereof, both said primary tumbler and said trapping tumbler being releasable by an auxiliary key so as to permit rotation of said plug to said unlocked position, and wherein said trapping tumbler is disposed so as not to engage said primary spline means or said secondary spline means during plug rotation between said locked and unlocked positions.