A parallelogram is a quadrilateral with opposite sides parallel (and therefore opposite angles equal). A quadrilateral with equal sides is called a rhombus, and a parallelogram whose angles are all right angles is called a rectangle. And, since a square is a degenerate case of a rectangle, both squares and rectangles are special types of parallelograms.
The polygon diagonals of a parallelogram bisect each other (Casey 1888, p. 2).
The angles of a parallelogram satisfy the identities
 |  |  |
(1)
|
 |  |  |
(2)
|
and
 |
(3)
|
A parallelogram of base 
and height 
has area
 |
(4)
|
The height of a parallelogram is
 |
(5)
|
and the polygon diagonals 
and 
are
 |  |  |
(6)
|
 |  |  |
(7)
|
 |  |  |
(8)
|
 |  |  |
(9)
|
(Beyer 1987).
The sides 
,
, 
, 
and diagonals 
,
of a parallelogram satisfy
 |
(10)
|
(Casey 1888, p. 22).
The area of the parallelogram with sides formed by the vectors 
and 
is
 |  |  |
(11)
|
 |  |  |
(12)
|
 |  |  |
(13)
|
where 
is the two-dimensional cross product and 
is the determinant.
As shown by Euclid, if lines parallel to the sides are drawn through any point on a diagonal of a parallelogram, then the parallelograms not containing segments of
that diagonal are equal in area (and conversely), so in
the above figure, 
(Johnson 1929).
The centers of four squares erected either internally or externally on the sides of a parallelograms are the vertices of a square (Yaglom 1962, pp. 96-97; Coxeter and Greitzer 1967, p. 84).
