Definition | Theorems and Postulates | Corresponding Angles Theorem

This is part of a collection of definitions of geometric theorems and postulates.

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The following section includes background information on angles. 

What Are Angles?

In this illustration two rays intersect at a point. Points A and B are on one ray and points B and C are on another ray. These two rays form an angle.

The angle is the corner sweep formed by the two rays. Rays BA and BC form Angle ABC. 

Point B is the vertex of the angle. The vertex is the corner formed by the angle.

Angles can also be formed by intersecting lines. In this case four angles are formed when lines intersect.

The same happens when two line segments intersect, as shown below.

Geometric shapes are sometimes defined by the number of angles they have.

Even circles can contain angles, as shown below.

How Are Angles Measured?

Angles are measured in degrees or in radians. In this illustration, Angle ABC is a 30° angle.

Angles are also measured in radians. Radian measures are useful for indicating the portion of a circle an angle sweeps.

Use a protractor to measure an angle.

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