See also: Path length (s), distance (r), height (h), radius (r),
diameter (d), circumference (u), wavelength (λ), u. v. a.
The length is a physical quantity that indicates the extent of physical objects and their distances from one another. The symbol for length is the lower case letter , its SI unit is the meter . A formula symbol is usually used for the length of paths and curves . In the SI system, the length is a basic quantity .
The length measurement is carried out by comparing the length to be measured with scales of known length. The length of the distances between two points or a scale is considered constant in classical physics. In the special theory of relativity , however, the length depends on the relative state of motion of the respective observer; one speaks of a relativistic length contraction .