# Length

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**Length** is the long dimension of any object. The length of a thing is the distance between its ends, its linear extent as measured from end to end. This may be distinguished from height, which is vertical extent, and **width** or **breadth**, which are the distance from side to side, measuring across the object at right angles to the length. In the physical sciences and engineering, the word "length" is typically used synonymously with "distance", with symbol or .

Length is a measure of one dimension, whereas area is a measure of two dimensions (length squared) and volume is a measure of three dimensions (length cubed). In most systems of measurement, length is a fundamental unit, from which other units are derived.

## Units of length

In the physical sciences and engineering, when one speaks of "units of length", the word "length" is synonymous with "distance". There are several units that are used to measure length. Units of length may be based on lengths of human body parts, the distance travelled in a number of paces, the distance between landmarks or places on the Earth, or arbitrarily on the length of some fixed object.

In the International System of Units (SI), the basic unit of length is the *metre* and is now defined in terms of the speed of light. The *centimetre* and the *kilometre*, derived from the metre, are also commonly used units. In U.S. customary units, English or Imperial system of units, commonly used units of length are the *inch*, the *foot*, the *yard*, and the *mile*.

Units used to denote distances in the vastness of space, as in astronomy, are much longer than those typically used on Earth and include the *astronomical unit*, the *light-year*, and the *parsec*.

Units used to denote microscopically small distances, as in chemistry, include the *micron* and the *ångström*.

## Length of moving rods

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While the length of a resting rod can be measured by direct comparison with a measuring rod, this comparison cannot be performed while the rod is moving. In this case we define its moving length as the distance between its two endpoints at a given instance.

If the world lines of the two endpoints of the rod expressed in the coordinates of an inertial reference frame are

and

- ,

then the length of the rod in this reference frame at the instance is

Since in special relativity the relation of simultaneity depends on the chosen frame of reference, the length of moving rods also depends.

## See also

- Distance
- Dimension
- Orders of magnitude (length)
- How round is your circle? Contains a chapter giving the history of length and angle measurement.
- Smoot

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