An inch (plural: inches; abbreviation or symbol: in or ″ – a double prime) is a unit of length in the imperial and United States customary systems of measurement. Historically an inch was also used in a number of other systems of units. Traditional standards for the exact length of an inch have varied in the past, but since July 1959 when the international yard was defined as 0.9144 metres, the international inch has been exactly 25.4 mm. There are 12 inches in a foot and 36 inches in a yard.