In Euclidean geometry, an isosceles trapezoid (isosceles trapezium in British English) is a convex quadrilateral with a line of symmetry bisecting one pair of opposite sides, making it automatically a trapezoid. Some sources would qualify this with the exception: "excluding rectangles." Two opposite sides (the bases) are parallel, and the two other sides (the legs) are of equal length (a property shared by the isosceles trapezoid and by the parallelogram). The diagonals are also of equal length.

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