An isosceles trapezoi (isosceles trapezium in British English) is a quadrilateral with a line of symmetry bisecting one pair of opposite sides, making it automatically a trapezoid. Two opposite sides are parallel, the two other sides are of equal length. The diagonals are of equal length. An isosceles trapezoid's base angles are congruent. Any quadrilateral with one axis of symmetry must be either an isosceles trapezoid or a kite.