Abstract: A key-operable lock having a releasable plug which is rotatable from a locked condition to an unlocked condition. The plug is removable from a fixed mounting member only in the unlocked condition and is blocked for removal in the locked condition by the combined effects of a master key-operable tumbler which is slideable in the plug and a boss which is fixed to the plug. In the locked condition, the boss and one end portion of the control tumbler extend outwardly from the plug to engage a fixed mounting member and the diametrically opposite end portion of the tumbler is inside of the plug. When a master key is inserted into the lock and the plug is rotated to the unlocked position both end portions of the control tumbler are inside of the plug; the boss is aligned with a slot of the mounting member and the plug is free to be removed from the fixed mounting member.