Quadrilateral, << `kwahd` ruh LAT uhr uhl, >> is the name given to a plane figure with four straight sides–that is, a four-sided polygon. A quadrilateral whose opposite sides are parallel is a parallelogram. The opposite sides and opposite angles of any parallelogram are equal. If the angles of a parallelogram are right angles, the figure is a rectangle. If all four sides are equal, the figure is a rhombus. A parallelogram that has four equal sides and four right angles is a square. The area, A, of any parallelogram with base b and altitude h is given in the formula A = bh. The altitude is the perpendicular distance between the base and the side opposite it.
A trapezoid is a quadrilateral with one set of parallel sides of unequal length. The trapezoid is isosceles if the nonparallel sides are equal.
