In geometry, a quadrilateral is a polygon with four sides or edges and four vertices or corners. Sometimes, the term quadrangle is used, for etymological symmetry with triangle, and sometimes tetragon for consistency with pentagon (5 sided), hexagon (6 sided) and so on. However, the term quadrangle could be considered incorrect, as the laws of Euclid state that an angle's degree measure must be less than 180, and so a concave quadrilateral has only 3 angles.Quadrilaterals are either simple (not self-intersecting) or complex (self-intersecting). Simple quadrilaterals are either convex or concave.
Convex quadrilaterals are further classified as follows:
Trapezium (British English) or trapezoid (Amer.): two opposite sides are parallel.
Isosceles trapezium (Brit.) or isosceles trapezoid (Amer.): two opposite sides are parallel, the two other sides are of equal length, and the two ends of each parallel side have equal angles. This implies that the diagonals are o