Fathom (fm)

Derived from Old English for outstretched arms, traditionally used for measuring water depth.

The fathom became an essential nautical unit for sounding ocean depths using weighted lead lines, allowing sailors to quickly gauge safe passage clearances beneath a ship’s hull. The six-foot fathom is the standard, but it was not always the only one. Shorter fathoms of five and five and a half feet are recorded in some trades and vessels, which is unsurprising given the unit began as the span of a particular man’s arms: a sounding line hauled in by hand was measured by the person hauling it. Standardisation came gradually, and British Admiralty charts were marked in six-foot fathoms long before every working sailor reckoned in them.