Two sets of contoured, flexible ribs allow the upper back to flex more than the lower, for exceptional comfort and support. One-piece molded polypropylene shell, a pneumatic cylinder. Chair swivels 360°. Pedestal base has a five-prong construction. Adjustable 18" diameter Chrome footring. Five 2" diameter, dual-wheel, hooded swivel casters. Optional 5-pack of fixed glides and 5-pack of soft-wheel casters are available.