Try this Drag the orange dots on each vertex
to reshape the parallelogram. Notice how the opposite sides remain parallel.

A parallelogram is a
quadrilateral with opposite sides
parallel.
But there are various tests that can be applied to see if something is a parallelogram.

It is the "parent" of some other quadrilaterals,
which are obtained by adding restrictions of various kinds:

A rectangle is a parallelogram but with all four interior angles fixed at 90°

A rhombus is a parallelogram but with all four sides equal in length

A square is a parallelogram but with all sides equal in length and all
interior angles 90°

A quadrilateral is a parallelogram if:

Both pairs of opposite sides are parallel. (By definition). Or:

Both pairs of opposite sides are congruent. If they are congruent, they must also be parallel. Or:

One pair of opposite sides are congruent and parallel. Then, the other pair must also be parallel.

Properties of a parallelogram

These facts and properties are true for parallelograms and the descendant shapes: square, rectangle and rhombus.

Base

Any side can be considered a base. Choose any one you like. If used to calculate the area (see below) the corresponding altitude must be used.
In the figure above, one of the four possible bases and its corresponding altitude has been chosen.

Altitude (height)

The altitude (or height) of a parallelogram is the perpendicular distance
from the base to the opposite side (which may have to be extended). In the figure above, the altitude corresponding to the base CD is shown.

Area

The area of a parallelogram can be found by multiplying a base by the corresponding altitude. See also Area of a Parallelogram

Opposite sides are
congruent (equal in length) and
parallel.
As you reshape the parallelogram at the top of the page, note how the opposite sides are always the same length.

Diagonals

Each diagonal cuts the other diagonal into two equal parts, as in the diagram below. See
Diagonals of a parallelogram for an interactive demonstration of this.

Interior angles

Opposite angles are equal as can be seen below.

Consecutive angles are always supplementary (add to 180°)