Parallelogram

Parallelogram, in plane geometry, a four-sided polygon, or quadrilateral, in which each side is equal in length to its opposite side, and the opposite sides are parallel to each other. Three special kinds of parallelogram are the square, in which all four sides are equal in length and meet at right angles; the rectangle, in which only opposite sides are equal in length but all sides meet at right angles; and the rhombus, in which all sides are equal in length but do not necessarily meet at right angles. Area is the product of one side, taken as base, times the shortest distance to the opposite side. See also Vector.