Inch

The inch (a Middle English word, from Old English ynce, from Latin uncia a twelfth part, from unus, una, unum one) is a short unit of length measure in several systems of measurement in common use today. These include the US Customary system and the Imperial system (now in use in the Commonwealth of Nations).

By convention, twelve inches make one foot--hence the inch's name.

One inch was originally the length of three barley corns, which form a distance approximating the breadth of the bridge of an adult human's nose. Today, in the United States of America, the US Customary inch is defined as precisely 2.54 centimeters, by law and international convention.