# Definition:Polygon/Internal Angle

< Definition:Polygon(Redirected from Definition:Internal Angle)

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## 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**.

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.

## Also see

- Results about
**internal angles**can be found**here**.

## Sources

- 1998: David Nelson:
*The Penguin Dictionary of Mathematics*(2nd ed.) ... (previous) ... (next):**interior angle**:**1.** - 2008: David Nelson:
*The Penguin Dictionary of Mathematics*(4th ed.) ... (previous) ... (next):**interior angle**:**1.** - 2014: Christopher Clapham and James Nicholson:
*The Concise Oxford Dictionary of Mathematics*(5th ed.) ... (previous) ... (next):**interior angle**(of a polygon)