Angles: Definition, Parts, and Notations

sides of an angle

Each of the rays that come together to form an angle

An angle consists of two rays or line segments that meet at a common endpoint. The common endpoint of these rays is called the vertex of the angle.

The two line segments or rays forming an angle divide the plane into three regions: the interior of the angle, the exterior of the angle, and the angle itself.

Each of the two sides of an angle is a line segment or ray that extends from the vertex to the edge of the angle.

When we label the sides of an angle, we use the vertex as the starting point and name the two rays that form the angle. For example, in the angle shown below, we can label the sides as BC and BA or as CB and CA:

“`
A
|
|
|
C——-B
“`

Here, side BC is also known as the terminal side, while side BA is also called the initial side. Side CB as initial side and CA as terminal side, would be the same sides, as they are just being named in opposite directions.

It is important to remember that the order in which we name the sides of an angle is important. When we refer to an angle as ∠ABC, side BC is always the side that extends from the vertex and intersects the other side or ray of the angle at point C. We must be consistent in the labeling of an angle’s sides to avoid any confusion.

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