In Euclidean geometry, a rectangle's a quadrilateral with four right angles. Equivalently, it's an equiangular quadrilateral, but it isn't necessarily equilateral. A rectangle with vertices ABCD would be denoted as . Properties All angles are 90 degrees. Opposite sides are equal in length. Opposite sides are parallel. Diagonals are equal in length and bisect each other. Area, perimeter, and other facts If a rectangle has length l and width w it's area A = lw perimeter P = 2l + 2w = 2(l + w… (More on Rectangle)