The Mul-T-Lock Captive Thumb-Turn Deadbolt

As you can see from the above animation the thumb-turn cannot be removed from the lock

without the use of a working key. Once the correct key is inserted into the keyhole above the

thumb-turn and rotated the thumb-turn can be removed. The lock can still be operated with the

use of the correct key.

Thus the lock is converted from a single cylinder deadbolt to a double cylinder deadbolt.