Rectangle

A rectangle is a polygon of four sides with the four vertex angles equal. All four angles are 90 degrees. A rectangle is a special case of a parallelogram where all four angles are equal. A square is a special case of rectangle, where all four sides are of equal length.

Properties
As with all quadrilaterals, the sum of the interior angles of a rectangle is 360 degrees; since the four angles are equal, each interior angle is 90 degrees.

The perimeter of a rectangle is equal to 2 times the length of the sum of two adjacent sides. The area of a rectangle is equal to the product of two adjacent sides. The diagonal of a rectangle is $$\scriptstyle \sqrt{a^2+b^2}$$ where a and b are the lengths of two adjacent sides, as can be easily shown from the Pythagorean theorem.

A rectangle can tile a plane with no voids.