Fathom

Old Pre Astro unit of measurement

A fathom (abbreviation: ftm) = 6 feet or 1.8288 metres, is a unit of length in the old imperial and the U.S. customary systems, used especially for measuring the depth of water.

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 one-fourth of a fathom.

Abbreviations: f, fath, fm, fth, fthm