The X-Chair is the most comfortable, ergonomic office chair we could find! I have had back issues for years, and I absolutely love the Dynamic Variable Lumbar support that separates the X-Chair from competitors. Not just a bar in your back, but a support that moves with you and meets you where you need it. X-Chair is also the only chair we have worked with that has seat depth adjustment to create a perfect sitting position. The X-3 has a high resiliency molded foam seat for comfort, covered in A.T.R. (advanced tensile recovery) fabric. The X-4 comes with your choice of Premium Leather or Brisa. The Premium Leather offers a familiar feel to other executive chairs. Brisa utilizes a gel polymer with a breakthrough ventilation system, all while feeling softer to the touch than leather or any other fabric on the market.