Drill bit shank shaft and shank, is usually of the same diameter. It is held usually in a three-jaw drill chuck. Bits of diameter too small to grip firmly can have straight shanks of larger diameter than the drill, which can be held firmly in a standard size collet or chuck. Large drill bits can have straight shanks narrower than the drill diameter so that they can be fitted in chucks not able to chuck the full diameter. Such a drill bit is called a reduced-shank or blacksmith's drill. For example, this allows a bit to be used in a pistol-grip drill's chuck.
Drill bit sizes size A (the smallest) to size Z (the largest). Although the ASME B94.11M twist drill standard, for example, lists sizes as small as size 97, sizes smaller than 80 are rarely encountered in practice. Number and letter sizes are commonly used for twist drill bits rather than other drill forms, as the range encompass the sizes for which twist drill bits are most often used. The gauge-to-diameter ratio is not defined by a formula, but is instead based on, but is not identical to, the Stubs Steel Wire Gauge, which originated in Britain during the 19th century. The accompanying graph,