Abstract: A lock which is rekeyable or programmable to operate with a different key from the exterior thereof without disassembling it or removing it from a lock assembly in which it is assembled is disclosed. The lock may be rekeyed through the keyway so that the lock appears from the exterior thereof to be a conventional lock, or from the end opposite the keyway from the interior of a door, or from a side of the lock which requires disassembly of the lock from its lock assembly. The lock includes structure defining a movable shear line that can be adjusted from the exterior of the lock without disassembling the lock itself. Such structure may comprise a rotatable cam and a pin tumbler and an axially adjustable set screw. Security of the lock is improved by selectively restricting or blocking access to the rekeying of the lock in the interior thereof with a gate or shutter, or various combinations of gates or shutters.