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The foot (standard symbol: ft) [1] [2] is a unit of length in the British imperial and United States customary systems of measurement. The prime symbol, ′, is commonly used to represent the foot. [3] In both customary and imperial units, one foot comprises 12 inches, and one yard comprises three feet.
The prime symbol ′ is commonly used to represent feet (ft), and the double prime ″ is used to represent inches (in). [2] The triple prime ‴, as used in watchmaking, represents a ligne (1 ⁄ 12 of a "French" inch, or pouce, about 2.26 millimetres or 0.089 inches). [3] Primes are also used for angles.
The international standard symbol for inch is in (see ISO 31-1, Annex A) but traditionally the inch is denoted by a double prime, which is often approximated by a double quote symbol, and the foot by a prime, which is often approximated by an apostrophe. For example; three feet, two inches can be written as 3 ′ 2″.
For measuring length, the U.S. customary system uses the inch, foot, yard, and mile, which are the only four customary length measurements in everyday use. From 1893, the foot was legally defined as exactly 1200 ⁄ 3937 m (approximately 0.304 8006 m). [13] Since July 1, 1959, the units of length have been defined on the basis of 1 yd = 0.9144 ...
A ruler, depicting two customary units of length, the centimeter and the inch. A unit of length refers to any arbitrarily chosen and accepted reference standard for measurement of length. The most common units in modern use are the metric units, used in every country globally. In the United States the U.S. customary units are also in use.
Hatch marks are frequently used as an abbreviation of some common units of measurement. In regard to distance, a single hatch mark indicates feet, and two hatch marks indicate inches. In regard to time, a single hatch mark indicates minutes, and two hatch marks indicate seconds.
The yard (symbol: yd) [3] [4] is an English unit of length in both the British imperial and US customary systems of measurement equalling 3 feet or 36 inches. Since 1959 it has been by international agreement standardized as exactly 0.9144 meter. A distance of 1,760 yards is equal to 1 mile. The US survey yard is very slightly longer.
historical definitions of the units and their derivatives used in old measurements; e.g., international foot vs. US survey foot. For some purposes, conversions from one system of units to another are needed to be exact, without increasing or decreasing the precision of the expressed quantity.