Protects post from nature"s elements. 4" x 4" post cap has 3-5/8" x 3-5/8" opening size, for posts measuring 3-1/2" x 3-1/2". 6" x 6" post cap has 5-5/8" x 5-5/8" opening size, for posts measuring 5-1/2" x 5-1/2".