Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. An isosceles trapezoid (isosceles trapezium in British English) is a quadrilateral with a line of symmetry bisecting one pair of opposite sides, making it automatically a trapezoid. Some sources would qualify all this with the exception: "excluding rectangles." Two opposite sides (bases) are parallel, the two other sides (legs) are of equal length. The diagonals are of equal length. An isosceles trapezoid''s base angles are equal in measure. Any quadrilateral with exactly one axis of symmetry must be either an isosceles trapezoid or a kite.