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The apex of a geometric figure is the point which is distinguished from the others by dint of it being furthest away from its base.

Not all figures have a discernible apex; for example, parallelograms, prisms and parallelepipeds do not.

Apex of Triangle


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.

Apex of Isosceles Triangle


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

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

Apex of Cone


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

Apex of Pyramid


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.

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.

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