# Definition:Polygon/Internal Angle

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## Contents

## Definition

The **internal angle** of a vertex of a polygon is the size of the angle between the sides adjacent to that vertex, as measured *inside* the polygon.

## Also known as

An **internal angle** is also known as an **interior angle**.

## Note

The vertices and the sizes of the internal angles of those vertices are frequently referred to by the same letter.

Thus **the angle of vertex $A$** is called **angle $A$** and denoted $\angle A$.

This is considered by some to be an abuse of notation but its convenience outweighs its disadvantages.