Measurement has been important ever since humans settled from nomadic lifestyles and started using building materials, occupying land and trading with neighbours.

And later, as society has become more technologically oriented, much higher accuracy of measurement is required in an increasingly diverse set of fields, from micro-electronics to interplanetary ranging.

[3] Under Einstein's special relativity, length can no longer be thought of as being constant in all reference frames.

In Euclidean geometry, length is measured along straight lines unless otherwise specified and refers to segments on them.

Pythagoras's theorem relating the length of the sides of a right triangle is one of many applications in Euclidean geometry.

If a long thin rectangle is stood up on its short side then its area could also be described as its height × width.

The volume of a solid rectangular box (such as a plank of wood) is often described as length × height × depth.

In the one-dimensional case, the Lebesgue outer measure of a set is defined in terms of the lengths of open intervals.

Concretely, the length of an open interval is first defined as so that the Lebesgue outer measure

The millimetre (mm), centimetre (cm) and the kilometre (km), derived from the metre, are also commonly used units.

[7] 1.609344 km = 1 miles Units used to denote distances in the vastness of space, as in astronomy, are much longer than those typically used on Earth (metre or kilometre) and include the astronomical unit (au), the light-year, and the parsec (pc).