# Definition:Cone (Geometry)/Apex

< Definition:Cone (Geometry)(Redirected from Definition:Apex of Cone)

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## Definition

Consider a cone consisting of the set of all straight lines joining the boundary of a plane figure $PQR$ to a point $A$ not in the same plane of $PQR$:

In the above diagram, the point $A$ is known as the **apex** of the cone.

## Also known as

The **apex** of a cone is also known as its **vertex**.

## Also see

## 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

- 1933: D.M.Y. Sommerville:
*Analytical Conics*(3rd ed.) ... (previous) ... (next): Chapter $\text {IV}$. The Ellipse: $1$. - 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**)