INCLUDES 4 DONUT BUMPERS TO HELP PROTECT DOORWAYS AND WALLS. ATTACHES TO BOTTOM OF POST. SET OF 4 CASTERS. REQUIRES THE USE OF A ""TIE BAR"" TO ASSURE RIGID CASTER ALIGNMENT. EVERY TWO RIGID CASTERS REQUIRE ONE TIE BAR. THE BAR SIZE MUST MATCH THE WIDTH OF THE SHELF.