The D-lock: the safest way to protect your bike against theft. D-locks are characterized by their U or D-shape and are therefore also called U-locks. They are made of hardened steel and can be locked with a key or a code. They are relatively heavy, large, and limited in reach, but they often come with a bracket that allows you to mount them to the frame of your bike. A small price to pay for optimal security of your bicycle against theft.