There are two yards (6 feet) in an imperial or U.S. fathom. Originally based on the distance between a man's outstretched arms, the size of a fathom has varied slightly depending on whether it was defined as a thousandth of an (Admiralty) nautical mile or as a multiple of the imperial yard. Formerly, the term was used for any of several units of length varying around 5–5+1⁄2 feet (1.5–1.7 m).
The name derives from the Old English word fæðm, corresponding to the old Frisian word "fadem" meaning embracing arms or a pair of outstretched arms. In Middle English it was fathme. A cable length, based on the length of a ship's cable, has been variously reckoned as equal to 100 or 120 fathoms. At one time, a quarter meant a fourth of a fathom.
Abbreviations: f, fath, fm, fth, fthm.