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 millimeter (symbol: mm) is a unit of length in the metric system, equal to one-thousandth of a meter (0.001 m). It is a standard unit used for measuring small distances and thicknesses, especially in engineering and manufacturing.

History/origin: The millimeter is derived from the meter, which was originally defined in 1793 during the French Revolution as one ten-millionth of the distance from the equator to the North Pole along a meridian. The metric system was created to provide a universal and standardized measurement system, and the millimeter emerged as a practical subunit of the meter for smaller-scale measurements.

Current use: The millimeter is widely used around the world, particularly in countries that have adopted the metric system. It is commonly used in mechanical engineering, design specifications, science, and everyday measurements, such as the thickness of paper, the diameter of wires, or dimensions in construction and manufacturing. In many contexts, it has replaced the inch due to the global standardization of the metric system.