What is a quadrilateral?

   Quadrilateral is the name given to a plane figure with four straight sides, that is, a four-sided polygon. A quadrilateral whose opposite sides lie parallel is a parallelogram. The opposite sides are also equal when they are parallel, and in that case, the opposite angles are equal, too. If the angles of a parallelogram are right angles, the figure is a rectangle. If the rectangle has equal sides, it is a square. The figure is a rhomboid if it is a parallelogram without right angles. When all the sides of a rhomboid are equal, it is a rhombus. The area, A, of any parallelogram with base b and altitude h is given in the formula A = bh.
   A trapezoid is a quadrilateral with one set of parallel sides of unequal length. The trapezoid is isosceles if the nonparallel sides are equal.

Quadrilaterals