This chair features a one-piece polypropylene shell with a tapered opening in back, this eliminates the pressure on the lower back while providing support. Choose your seat size: A or A+. The A+ offers a wider more accommodating seat. The progressive waterfall front seat edge promotes good circulation and relieves pressure points. The sturdy 5-star base has a 24" diameter, swivels 360 degrees, and features durable 2" dual casters. With an adjustable pneumatic height of 24"-34", and simple turn handle to adjust the 18"-foot ring this chair is comfortable at any height.