Abstract: In a cylinder lock mechanism of the type in which longitudinally movable pin tumblers are housed in a normally freely rotatable lock plug which is coupled to a locking member by means of a rocking lever responding to correct alignment of the tumblers, the resistance of the lock to picking, and to fouling and corrosion may be improved, and the structure made more compact, by making the lever part of a three part linkage, the lever being pivotless and actuated by the tumblers by a laterally movable diaphragm in the lock plug and acting upon a laterally movable clutch dog to couple the locking member to the plug. A decombinating cam is provided to ensure disengagement of the diaphragm from the tumblers on removal of a key from the mechanism, and provision may be made positively to lock the locking member against rotation except when the mechanism is freed for rotation by insertion of a correct key.