# Definition:Triangle (Geometry)/Isosceles/Apex

< Definition:Triangle (Geometry) | Isosceles(Redirected from Definition:Apex of Isosceles Triangle)

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## Definition

The vertex opposite the base of an isosceles triangle is called the **apex** of the triangle.

In the above diagram, $A$ is the **apex**.

## Also known as

The **apex** of an isosceles triangle is also called variously:

**head angle****vertex**

Neither of these terms is endorsed on $\mathsf{Pr} \infty \mathsf{fWiki}$ for obvious reasons.

## Linguistic Note

The plural of **apex** is **apices**, which is pronounced ** ay-pi-seez**.

The form **apexes** can often be seen, but this is technically incorrect.

Compare **vertex**, whose plural is **vertices**.

Hence the colloquial phrase **base over apex** as the description of a particularly flamboyant physical tumble.