Abstract: Pivotable tumblers mounted to a cylindrical plug prevent rotation of the plug unless a correct key in a keyway moves the tumblers to an unlocked position. If the tumblers extend too far into the keyway, pawls mounted in the body and acting on steps of the plug prevent rotation of the plug to an unlocked position. If the tumblers do not extend for enough into the keyway, the tumblers jam up against a reentrant shoulder of the body preventing the plug from reaching the unlocked position. A security keyway closure or door cams off a race in the body and closes the keyway with rotation of the plug. A slot in the key passes the door during operation.