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In Euclidean geometry, a parallelogram is a (non self-intersecting) quadrilateral with two pairs of parallel sides. The opposite or facing sides of a parallelogram are of equal length and the opposite angles of a parallelogram are of equal measure. The congruence of opposite sides and opposite angles is a direct consequence of the Euclidean parallel postulate and neither condition can be proven without appealing to the Euclidean parallel postulate or one of its equivalent formulations. The three-dimensional counterpart of a parallelogram is a parallelepiped. A quadrilateral with one pair of parallel sides is a trapezoid in American English or a trapezium in British English. The etymology (in Greek παραλληλ-όγραμμον, a shape "of parallel lines") reflects the definition. ==Special cases== *Rhomboid – A quadrilateral whose opposite sides are parallel and adjacent sides are unequal, and whose angles are not right angles〔 http://www.cimt.plymouth.ac.uk/resources/topics/art002.pdf〕 *Rectangle – A parallelogram with four angles of equal size *Rhombus – A parallelogram with four sides of equal length. *Square – A parallelogram with four sides of equal length and angles of equal size (right angles). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Parallelogram」の詳細全文を読む スポンサード リンク
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