An inch (plural: inches; abbreviation or symbol: in or ″ – 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. There are 36 inches in a yard and 12 inches in a foot. Corresponding units of area and volume are the square inch and the cubic inch.
The inch is a commonly used customary unit of length in the United States,Canada, and the United Kingdom. For the United Kingdom, guidance on public sector use states that since 1 October 1995, without time limit, that the inch (along with the mile, yard and foot) is to be used as a primary unit for road signs and related measurements of distance and speed and may continue to be used as a secondary or supplementary indication following a metric measurement for other purposes.
From July 1, 1959, the United States and countries of the British Commonwealth defined the length of the international yard to be exactly 0.9144 metres. Consequently, the international inch is defined as exactly 25.4 millimetres. This creates a slight difference between the international units and American surveyor's units which are described in the article on the foot.