The foot is a unit for measuring length. It is one of the Imperial units and US customary units. The shortest way of writing the unit "foot" is by the abbreviation "ft" (or "ft."), or by a prime symbol ( ′ ).
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The foot was a common unit of measurement throughout Europe. It often differed in length not only from country to country but from city to city. Because the length of a foot changed between person to person, measurements were not even consistent between two people, often requiring an average. Henry I of England was attributed to passing the law that the foot was to be as long as a person's own foot. This was one of the first times a standard unit of measurement was put into place.
In 1959, the United States, Canada, New Zealand, South Africa, Australia and the United Kingdom all signed a treaty known as the 'International Yard and Pound Agreement'. This standardised the avoirdupois weight and length of all nations. Those standards are still officially used in both the Imperial system and US customary units.
Road signs with height restrictions in the United States and the United Kingdom are in feet and inches. They are less commonly seen in Canada and Ireland. In the US and the UK, people usually measure a person's height in feet and inches. If a person is "five foot eleven", this means that person is 5 feet and 11 inches, or about 180 centimeters. The height is written 5′ 11″