An inch (plural: inches; symbol or abbreviation: in or, sometimes, ″ – a double prime) is the name of a unit of length in a number of different systems, including Imperial units, and United States customary units. Its size can vary from system to system. There are 36 inches in a yard and 12 inches in a foot. A corresponding unit of area is the square inch and a corresponding unit of volume is the cubic inch. The inch is usually the universal unit of measurement in the United States, and is occasionally used in Canada and the United Kingdom, despite the introduction of the Metric system in the 1960s in the latter country.
In 1958 the United States and countries of the Commonwealth of Nations defined the length of the international yard to be 0.9144 meters. Consequently, the international inch is defined to be equal to 25.4 millimeters.