A rectangle is a quadrilateral on a plane having all interior angles equal to 90° (all interior angles are right angles). An equivalent definition is: rectangle is a parallelogram having at least one interior angle equal to 90°. (Note that if a parallelogram has one interior angle equal to 90°, then all its interior angles are equal to 90°). These and other properties of rectangles are discussed in lessons under this topic. So, a rectangle is a special case of a parallelogram. A square is a special case of a rectangle. Handy formulas: Perimeter = 2*(width+height), area = width*height, where width and height are sizes of a rectangle sides.