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Definition

A parallelogram is a quadrilateral whose opposite sides are parallel to each other, and whose sides may or may not all be the same length.

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.

Altitude

An altitude of a parallelogram is a line drawn perpendicular to its base, through one of its vertices to the side opposite to the base (which is extended if necessary).

In the parallelogram above, line $DE$ is an altitude of the parallelogram $ABCD$.

The term is also used for the length of such a line.

Also see

• Results about parallelograms can be found here.

Four day, five day marathon
We're moving like a parallelogram