Though hex bolts go by many names the characteristic that they all share is a hexagonal head, designed to be driven by a wrench. They all are commonly referred to as both bolts and screws. They are unique because of the threading. These bolts are partially threaded with a shoulder beneath the head. Tap Bolts are fully threaded up to the hexagonal head. The unique characteristic of a hex bolt is the design of the head, from which they get their name. The "flange" of the bolt is a built in (integral) washer. In Imperial sizes, this washer is serrated to act as a lock washer.