Definition:Polygon/Base

Definition

For a given polygon, any one of its sides may be temporarily distinguished from the others, and referred to as the base.

It is immaterial which is so chosen.

The usual practice is that the polygon is drawn so that the base is made horizontal, and at the bottom.

For polygons of particular types, the following specific definitions can be made:

For a given triangle, one of the sides can be distinguished as being the base.

It is immaterial which is so chosen.

The usual practice is that the triangle is drawn so that the base is made horizontal, and at the bottom.

In the above diagram, it would be conventional for the side $AC$ to be identified as the base.

The base of an isosceles triangle is specifically defined to be the side which is a different length from the other two.

In the above diagram, $BC$ is the base.

In a given parallelogram, one of the sides is distinguished as being the base.

It is immaterial which is so chosen, but usual practice is that it is one of the two longer sides.

In the parallelogram above, line $AB$ is considered to be the base.