The inch is a unit of length in the Imperial system and the United States customary system. The abbreviation for inches is in or ". There are 12 inches in a foot. One inch is equal to 2.54 centimetres.
The word "inch" came from Middle English unche, which came from Old English ynce, from Latin uncia meaning "a twelfth part".
The inch was originally defined as 3 barleycorns. The inch was finally standardised in the International Yard and Pound Treaty in 1959 between the United States, the United Kingdom, South Africa, Australia, New Zealand and Canada. The international yard was made equal to 0.9144 metres. From this, subdivisions and multiples of the yard were specifically defined.
In Britain and the United States, people use inches more than they use millimetres or centimetres. In the rest of the world, international units are almost always used. The inch is not used by scientists.
In the United Kingdom, road signs that show how high a vehicle can be in order to pass through a tunnel are required to be in feet and inches. Theme parks and drive thru signs usually show it in metres. People regularly measure their height in feet and inches. Official medical records, however, are required to record people's height in metric measurements only.
In Canada, a mix of centimetres and inches are used in height. Older generations, especially, use Imperial units. A lot of exposure to Americanized phrases leads to younger generations often having a good understanding of both the Imperial and metric systems.
In the United States, height is always in feet and inches. Science is the only field to use metric measurements.
Other Commonwealth countries, including Ireland, Australia, New Zealand, South Africa and Jamaica use inches to varying degrees. From every day use to exclusively the older community.