Quadrilateral, plane figure (that is, a shape drawn on a flat surface) bounded by four straight lines called its sides. It has four corners, called vertices, and the sum of their internal angles is 360°. If these angles are all right angles, the quadrilateral is called a rectangle, and opposite sides have equal lengths. If all four sides are equal the quadrilateral is a rhombus, and opposite angles are equal. A square is both a rectangle and a rhombus, so all its angles and all its sides are equal.
A rectangle, a rhombus, and a square are all examples of parallelograms, quadrilaterals in which opposite sides are parallel.
