Definition: An inch (symbol: in) is a unit of length in the imperial and US customary systems of measurement. An inch was defined to be equivalent to exactly 25.4 millimeters in 1959. There are 12 inches in a foot and 36 inches in a yard.

History/origin: The term "inch" was derived from the Latin unit "uncia" which equated to "one-twelfth" of a Roman foot.
There have been a number of different standards for the inch in the past, with the current definition being based on the international yard. One of the earliest definitions of the inch was based on barleycorns, where an inch was equal to the length of three grains of dry, round barley placed end-to-end. Another version of the inch is also believed to have been derived from the width of a human thumb, where the length was obtained from averaging the width of three thumbs: a small, a medium, and a large one.

Current use: The inch is mostly used in the United States, Canada, and the United Kingdom. It is also sometimes used in Japan (as well as other countries) in relation to electronic parts, like the size of display screens.

Definition: A foot (plural: feet, symbol: ft) is a unit of length in the imperial and U.S. customary systems of measurement. One foot is equal to 12 inches and is exactly defined as 0.3048 meters. It is commonly used to measure height, distance, and dimensions in non-metric countries.

History/origin: The foot has ancient origins and was historically based on the length of a human foot. It was used in many different cultures, including the Romans, Greeks, and Chinese, with varying definitions. The current international foot was defined in 1959 through an agreement between English-speaking nations, standardizing it to exactly 0.3048 meters.

Current use: The foot is primarily used in the United States, as well as in the United Kingdom and Canada for certain applications. It is widely used in construction, aviation, and real estate to measure height, elevation, and floor plans. Despite the global shift to the metric system, the foot remains prevalent in everyday use in countries that have not fully adopted metric standards.