In geometry, a **parallelogram** is a quadrilateral with two sets of parallel sides. The opposite sides of a parallelogram are of equal length, and the opposite angles of a parallelogram are congruent. The three-dimensional counterpart of a parallelogram is a parallelepiped.

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- The two parallel sides are of equal length.
- The area, , of a parallelogram is where is the base of the parallelogram and is its height.
- The area of a parallelogram is twice the area of a triangle created by one of its diagonals.
- The area is also equal to the magnitude of the vector cross product of two adjacent sides.
- The diagonals of a parallelogram bisect each other.
- It is possible to create a tessellation with any parallelogram.
- The parallelogram is itself a special case of a trapezoid.

In a vector space, addition of vectors is usually defined using the parallelogram law. The parallelogram law distinguishes Hilbert spaces from other Banach spaces.

To prove that the diagonals of a parallelogram bisect each other, first note a few pairs of equivalent angles:

Since they are angles that a transversal makes with parallel lines and .

Also, since they are a pair of vertical angles.

Therefore, since they have the same angles.

From this similarity, we have the ratios

Since , we have

- .

Therefore,

bisects the diagonals and .

The area formula,

can be derived as follows:

The area of the parallelogram to the right (the blue area) is the total area of the rectangle less the area of the two orange triangles. The area of the rectangle is

and the area of a single orange triangle is

Therefore, the area of the parallelogram is

- Arnone, Wendy. 2001.
*Geometry for Dummies*. Hoboken, NJ: For Dummies (Wiley). ISBN 0764553240 - Hartshorne, Robin. 2002.
*Geometry: Euclid and Beyond*. Undergraduate Texts in Mathematics. New York: Springer. ISBN 0387986502 - Leff, Lawrence S. 1997.
*Geometry the Easy Way*. Hauppauge, NY: Barron’s Educational Series. ISBN 0764101102 - Stillwell, John. 2005.
*The Four Pillars of Geometry*. Undergraduate Texts in Mathematics. New York: Springer. ISBN 0387255303

All links retrieved January 14, 2019.

- Weisstein, Eric W. Parallelogram
*MathWorld—A Wolfram Web Resource.* - Parallelogram: Properties, Shapes, Diagonals and Area
- Parallelogram
*Math Open Reference*. (Definition and properties.) - Area of a Parallelogram
*Math Open Reference*. - Area of Parallelogram
*Cut The Knot*. - Equilateral Triangles On Sides of a Parallelogram
*Cut The Knot*. - Parallelogram: Online Quiz
*kwizNET Learning System*.

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