B - Backrest
Backrest - The upper part of a seat. The backrest of front seats usually tilt in order to help the occupant find the most comfortable driving position. Some also include lumbar adjustment. (See also Lumbar support).
The rear-seat backrests of some cars can also be tilted for comfort and many cars have rear backrests that can split and fold to increase the capacity of the boot. Some have rear backrests that can only be released from inside the boot, to improve security.