# Definition:Pyramid/Apex

< Definition:Pyramid(Redirected from Definition:Apex of Pyramid)

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## Definition

The vertex of a pyramid which is the common vertex of its triangular faces is called the **apex** of the pyramid.

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

## Also known as

The **apex** of a pyramid is seen in some sources as **vertex**.

## Also see

- Definition:Apex of Tetrahedron:
**all**the faces of a tetrahedron are triangular, so this definition cannot apply as it stands.

## 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.

## Sources

- 1998: David Nelson:
*The Penguin Dictionary of Mathematics*(2nd ed.) ... (previous) ... (next):**apex**(plural**apices**) - 2008: David Nelson:
*The Penguin Dictionary of Mathematics*(4th ed.) ... (previous) ... (next):**apex**(plural**apices**)