inches

The inch is a unit of length used in the imperial and United States customary measurement systems. It is defined as exactly 25.4 millimeters and is commonly abbreviated in, with the technical symbol for inches being the double prime ″. In this system, 12 inches make up a foot, and 36 inches make up a yard; 1 mile equals 63,360 inches.

Historically, the inch derives from the Roman uncia (one twelfth of a foot). Over centuries, various regions used different inch sizes, but the modern inch was standardized through international agreements in the 20th century. In 1959, an agreement among the United States and several English-speaking countries defined the inch as exactly 25.4 millimeters, aligning it with the metric system for precision and interoperability.

In common use, inches appear in a variety of contexts, such as the dimensions of small objects, tools, and hardware, as well as screen sizes (diagonal measurement) for televisions and computer monitors. In measurement practice, inches are often expressed in fractions (for example, 1/2 in, 1/4 in, 1/8 in, and so on) or, in some technical contexts, with the decimal form of inches. The inch remains widespread in the United States and in some industries worldwide, though many countries rely primarily on metric units for everyday measurement. In scientific disciplines, the meter and other SI units are standard, with inches used mainly for reference or compatibility with non-scientific contexts.