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The Fathom (symbol fath) is a unit of length in the imperial and U.S. customary systems of measurement, defined as exactly 6 feet (ft). This is equivalent to 2 yards or 1.8288 meters (m). It is almost exclusively used in maritime contexts for measuring the depth of water.

The word "fathom" originates from the Old English word fæthm, meaning "a pair of outstretched arms" or "embrace". Historically, the length was derived from the distance between the fingertips of a man's outstretched arms, a practical method for sailors measuring the length of a sounding line (a rope with a weight used to measure water depth) as they hauled it in. This naturally variable measure was eventually standardized to precisely 6 feet. For centuries, it was the primary unit for depth soundings on nautical charts and in maritime operations in English-speaking countries and beyond. While its official use has declined with metrication, it remains a significant unit in nautical tradition.

The fathom's use is strongly tied to the sea:

• Hydrography & Navigation: Measuring water depth (soundings) on nautical charts. Many charts, particularly older ones or those used in the US and UK, still use fathoms. Contour lines showing seabed depth are often called "fathom lines".
• Nautical Terminology: Used historically to measure the length of ropes and anchor cables (e.g., a "cable length" was often defined as 100 or 120 fathoms).
• Commercial Fishing: Sometimes used by fishermen to describe the length of nets or fishing lines, or the depth at which they are fishing.
• Literature & Figurative Language: Frequently appears in maritime literature (e.g., "full fathom five" from Shakespeare's The Tempest). Metaphorically, "to fathom" means to comprehend or get to the bottom of something complex.
• Historical Land Measurement: Very occasionally used in historical land surveying or mining contexts, but this was rare compared to its maritime use.