**A four-sided polygon is known as a quadrilateral.** There are various types of quadrilaterals, but the most recognized are the square, rectangle, rhombus, trapezoid and the parallelogram. Quadrilaterals are always flat shapes that have four straight edges and four corners or vertices.

Quadrilaterals are considered simple if no self-intersections occur, or complex if self-intersection does occur. Depending on the shape, simple quadrilaterals are termed either concave or convex. Simple quadrilaterals have interior angles whose sum equals 360 degrees of arc, while complex quadrilaterals have a total of 720 degrees of arc when all four angels on both sides of the intersection are combined.