Fathom Fathom A fathom is a unit of length in the imperial and the U.S. customary systems equal to , used especially for measuring the depth of water. There are two yards (6 feet) in an imperial fathom. Originally the span of 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 . The name derives from the Old English word "fæðm", cognate
Fathom (trademark). The record made by such a device is a fathogram. A fathom line or fathom curve, a usually sinuous line on a nautical chart, joins all points having the same depth of water, thereby indicating the contour of the ocean floor. The components of a commercial fisherman's setline were measured in fathoms. The rope called a "groundline", used to form the main line of a setline, was usually provided in bundles of 300 fathoms. A single 50-fathom skein () of this rope was referred to as a "line". Especially in Pacific coast fisheries the setline was composed of units