The perimeter of a parallelogram is equal to the sum of the four lengths that make up the parallelogram. Instead of adding up all four sides, a person can find the perimeter by adding the width and the height and then multiplying the sum by two.
The perimeter of any polygon, not just a parallelogram, is equal to the sum of the length of the sides. A polygon by definition is a two-dimensional shape, contained in the same plane, constructed of straight lines, that has three or more sides. A parallelogram is a polygon but has exactly four sides, with each set of sides parallel and the same length.
A parallelogram is easier to solve the perimeter for than many polygons because of the fact that parallelograms have two sets of equal sides. This allows for the simpler math solution of adding the width and the height and multiplying by two. Squares and rhombuses are also parallelograms and polygons, except their four sides are all exactly the same length. To determine the perimeter of a square or rhombus, one can find one side length and multiply by four. The only difference between a square and a rhombus is that a square must have four 90-degree angles, while the angles of a rhombus need not equal 90 degrees.