A foot (plural: feet or foot; symbol or abbreviation: ft or, sometimes, ′ – a prime) is a unit of length, in a number of different systems, including English units, Imperial units, and United States customary units. Its size can vary from system to system, but in each is around a quarter to a third of a metre. The most commonly used foot today is the international foot. There are 3 feet in a yard and 12 inches in a foot.