# Quadrilaterals

A polygon that has four sides is called a quadrilateral.

The basic elements defining a quadrilateral are its four vertices, its four sides and its four angles.

**1. **If A, B, C and D are four points in a plane such that no three of them are collinear and the line segments AB, BC, CD and DA do not intersect except at their end points, then the figure formed by these four lines is called the quadrilateral ABCD.

**2.** A quadrilateral has four vertices (A, B, C and D), four sides (AB, BC, CD and AD), four angles ( A, B, C and D) AC and BD are the two diagonals.

**3.** In a concave quadrilateral, the measures of all angles are less than 180Â°. In a convex quadrilateral, the measure of one of the angles is more than 180Â°.

The sum of the angles of a quadrilateral is 360Â° or 4 right angles.