A thumbscrew is a type of screw drive with either a tall head and ridged or knurled sides, or a key-like flat sided vertical head. They are intended to be tightened and loosened by hand, and not found in structural applications. They are sometimes also cut for Phillips head and/or slotted screwdrivers as well as having the knurl for finger grip. ASME 18.6.8 covers dimensions for Type A, shoulder under the head, regular and heavy along with Type B, without shoulder, regular and heavy.