Inch (in)

• Definition: A unit of length in the Imperial and US Customary systems, defined since 1959 as exactly 25.4 mm.
• History: Originally based on human anatomy, historically defined as the width of an adult thumb or the length of three consecutive barleycorns.
• Common Uses: Screen sizes (TVs, smartphones), tire diameters, plumbing pipe sizes, and everyday measurements in the US and UK.

Millimeter (mm)

• Definition: A unit of length in the metric system equal to one-thousandth of a meter, 1 mm = 0.001 m.
• History: Defined during the French Revolution in the late 18th century when France adopted the metric system based on decimal divisions of the meter.
• Common Uses: Measuring small distances, thickness of materials (like paper or smartphone screens), rainfall amounts, and mechanical engineering tolerances.