# Angle - Units Of Measurement Of An Angle, Geometric Characteristics Of Angles

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An angle is a geometric figure created by two line segments that extend from a single **point** or two planes which extend from a single line. The size of an angle, measured in units of degrees or radians, is related to the amount of **rotation** required to superimpose one of its sides on the other. First used by ancient civilizations, angles continue to be an important tool to science and industry today.

The study of angles has been known since the time of the ancient Babylonians (4,000-300 B.C.). These people used angles for measurement in many areas such as construction, commerce, and **astronomy**. The ancient Greeks developed the idea of an angle further and were even able to use them to calculate the circumference of **Earth** and the **distance** to the **moon**.

A geometric angle is formed by two lines (rays) that intersect at a common endpoint called the vertex. The two rays are known as the sides of the angle. An angle can be specified in various ways. If the vertex of an angle is at point P, then the angle could be denoted by *L*P. It can be further described by using a point from each ray. For example, the angle *L*OPQ would have the point O on one ray, a vertex at point P, and the point Q on the remaining ray. An angle can also be denoted by a single number or character which is placed on it. The most common character used is the Greek letter θ (theta).

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