Height, depth, width, thickness, distance, diameter, perimeter, radius. We have been measuring these everyday quantities since childhood, for example with a ruler or tape measure.

There are many units of length: cubit, fathom, yard, chain, furlong, inch, arc, foot, mile, light year... We could go on and on. The introduction of the metric system has standardised the definition of length internationally. Its unit of measurement, the metre, was initially related to the size of the Earth, and the metre was used as the basis for the creation of the metre standard, made of an alloy of platinum and iridium. The length of the line between the two ends of the etalon is one metre. Later, its definition was refined to make it reproducible: one metre is the distance travelled by light in vacuum in 1/299792458th of a second.

Measuring instruments, measuring devices, measures work on the principle of comparison. Accurate measurements can be obtained by comparing smaller units of the meter. In technical specifications, all measurements must be given in millimetres, but note that rulers and measuring tapes are labelled in centimetres. In everyday life we use inches, 1inch=2.54cm.

We also meet signs marking size in many places in everyday life. Traffic restriction signs mark, e.g., the height of tunnels, or the distance to follow for drivers. In catalogues, we are informed about the dimensions of household appliances, furnishings, and utility items. Although we choose our clothes based on size, the use of custom-sized clothes is also common. As for the length of shoes, size 40 (UK 6.5), for example, is about 25 cm.