parallelogram, closed plane figure bounded by four line segments, or sides, with opposite pairs of sides parallel and equal in length. The rhombus, rectangle, and square are special types of parallelograms. Any side of a parallelogram is a base; an altitude is the perpendicular distance from a base to the opposite parallel side. The area of a parallelogram is equal to the product of the lengths of its base and altitude. The diagonals of a parallelogram, connecting opposite vertices, bisect one another; either diagonal divides the parallelogram into two congruent triangles.
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