When we think of a room, its walls and floors, a news paper, a note book, the computer screen, the surfaces of a rectangular box etc, we see flat surfaces which are rectangular. The rectangular surface is the one which has four sided and the sides are straight. So we can see that the rectangular surfaces are those which are bounded by four sides. All four sided figures are not rectangles. The four sided figures called quadrilaterals are classified into different types according to the sides and the angles. In this section let us see the definition of quadrilaterals and the other types of quadrilaterals and their properties.