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The length of a line segment is the distance between its endpoints. Length may be measured in meters, yards, abstract units, etc. For example, in the following diagram
 is one unit long,  is two units long,  is a fifth of a unit,  is four fifths of a unit,  is three units, etc.
The concept of length as defined above is a special case of a general concept called measure.
In two-dimensional space, length usually goes along the  axis while height goes along the  axis. The same holds in three-dimensional space.
For triangles, pentagons, and higher  -gons, it is customary to refer to the length of any of its sides as the length of the polygon. The length of a circle's side is called its circumference.
In set theory, length refers to the number of elements a set (or one-dimensional array) has. This is also known as cardinality. For example,
 has length  .
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