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In Euclidean plane geometry, a rectangle is any quadrilateral with four right angles. The term "oblong" is occasionally used to refer to a nonsquare rectangle.^{[1]}^{[2]} A rectangle with vertices ABCD would be denoted as ABCD.
A socalled crossed rectangle is a crossed (selfintersecting) quadrilateral which consists of two opposite sides of a rectangle along with the two diagonals.^{[3]} Its angles are not right angles. Other geometries, such as spherical, elliptic, and hyperbolic, have socalled rectangles with opposite sides equal in length and equal angles that are not right angles.
Rectangles are involved in many tiling problems, such as tiling the plane by rectangles or tiling a rectangle by polygons.
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A rectangle is a special case of a parallelogram, whose opposite sides are equal in length and parallel and connecting sides are perpendicular to each other.
A parallelogram, and hence also a rectangle, is a special case of a trapezium (known as a trapezoid in North America), which has at least one pair of parallel opposite sides.
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