D - Deadlocks
Deadlocks - Once activated, deadlocks can only be re-opened with the car's remote control or key. It is sometimes called double-locking as you have to press the 'plipper' on the key twice to activate the lock. Deadlocks prevent thieves from reaching in to open the door after smashing the window.
Mercedes-Benz is one manufacturer that refuses to fit or offer deadlocks on safety grounds, claiming they could trap occupants in a car following a collision.