Quadrilateral

A quadrilateral is a polygon with four sides.

As a consequence of the definition, the sum of the angles in a quadrilateral equals 360^o, and all quadrilaterals are capable of tessellating an infinite plane.

Classification of Quadrilaterals

The classification of quadrilaterals is complex, as there are several intersecting sub-categories:

• A (British) trapezium or (American) trapezoid has one pair of opposite sides parallel. A version of this called the isosceles trapezium has the other pair of opposite sides equal
• A parallelogram has two pairs of opposite sides parallel (and equal)
• A rectangle has four right angles
• A rhombus has four equal sides
• A kite has two pairs of adjacent equal sides
• A re-entrant quadrilateral has one internal angle greater than 180^o; this may be contrasted with a convex quadrilateral in which all internal angles are less than 180^o
• An arrowhead is a re-entrant kite
• A square is both a rectangle and a rhombus