Abstract: In a security door lock a pivoted detent deadlocks the bolt by engagement of a stump in pockets provided in the bolt. The detent is normally held in its locking position by a cam surface on the thrower and by a pack of detainers. Rotation of the correct key aligns notches in the detainers with a probe on the detent and also positions a second cam surface of the thrower adjacent to the detent, so the detent can pivot to its unlocking position under the action of a spring, and thereby free the bolt. After the bolt is shifted the final rotation of the key re-engages the cam surface against the detent to drive the detent positively back to its locking position. If required, the bolt can be double-thrown by a second ("master") key which lifts the detainers to higher positions so that second notches in the detainers now align with the detent probe. This higher lifting of the detainers also lifts a dogging slide so that the stump of the latter can pass over a projection on the bolt tail.