<Li> Trapezium (UK) or trapezoid (US): at least one pair of opposite sides are parallel . Trapezia (UK) and trapezoids (US) include parallelograms . </Li> <Li> Isosceles trapezium (UK) or isosceles trapezoid (US): one pair of opposite sides are parallel and the base angles are equal in measure . Alternative definitions are a quadrilateral with an axis of symmetry bisecting one pair of opposite sides, or a trapezoid with diagonals of equal length . </Li> <Li> Parallelogram: a quadrilateral with two pairs of parallel sides . Equivalent conditions are that opposite sides are of equal length; that opposite angles are equal; or that the diagonals bisect each other . Parallelograms include rhombi (including those rectangles we call squares) and rhomboids (including those rectangles we call oblongs). In other words, parallelograms include all rhombi and all rhomboids, and thus also include all rectangles . </Li> <Li> Rhombus or rhomb: all four sides are of equal length . An equivalent condition is that the diagonals perpendicularly bisect each other . Informally: "a pushed - over square" (but strictly including a square, too). </Li>

Quadrilateral with 2 pairs of equal sides and 2 pairs of opposite angles that are equal