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Parallelogram
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. 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 sides equal in length
 A square is a parallelogram but with all sides equal in length and all angles fixed at 90°
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 
Perimeter 
The distance around the parallelogram. The sum of its sides. See also Perimeter of a Parallelogram 
Opposite sides 
Opposite sides are congruent (equal in length).
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°)
For more on both these properties, see
Interior angles of a parallelogram.

Parallelogram inscribed in any quadrilateral
If you find the
midpoints
of each side of any
quadrilateral,
then link them sequentially with lines, the result is always a
parallelogram.
This may seem counterintuitive at first, but see
Parallelogram inscribed in any quadrilateral
for an animated exploration of this fact.
Related polygon topics
General
Types of polygon
Area of various polygon types
Perimeter of various polygon types
Angles associated with polygons
Named polygons
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