An inch (in.) is a unit of measurement used to express length or distance in the imperial system. Other units in this system include feet (ft.) and yards (yd.), which also express length or distance, and pounds (lb.), which express weight.
1 yard = 3 feet = 36 inches. Converter to the metric system, 1 inch = 25.4 millimeters.

There are multiple theories about the origin of the term. One theory is that it comes from dividing one foot (which is based on the length of a foot, from the end of the toenail to the heel) into twelve parts, each of which is the width of a thumb, and is referred to as an “inch.” Another is that it is the size of three barleycorns in a row.

Today, the standard unit for measuring length used around the world is the meter (m.). This unit was created as part of the “metric system,” which was the result, in 1875, of an effort to standardize units internationally. After Japan signed the treaty of meter in 1885, the metric system came to be followed for measurements used in transactions and for other official purposes. Even today, however, America and a small number of other countries still primarily use the imperial system. In daily life, the inch can be seen used as the conventional unit of measurement for bicycle frames, bicycle tires, television screens, shoe size, floppy discs, CDs, and other media. In sports, golf and American football use the yard to express distance. As these examples show, the use of the imperial system to express length remains surprisingly common even today.