Definition:Triangle (Geometry)/Apex

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Having selected one side of a triangle to be the base, the opposite vertex to that base is called the apex.

In the above diagram, if $AC$ is taken to be the base of $\triangle ABC$, then $B$ is the apex.

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.