What many modern chairs sadly lack. A stiffness or even bulge in the chair at the lower back area, which serves to cushion the lumbar area of the spine (where it curves inward) so that sitting back in the chair does not place undue stress on the spinal curve. Most high-end office chairs, auto seats, etc. have adjustable lumbar support, via a movable or inflatable pad. If you develop pain in the lower back after sitting for a long period of time, it's likely that your chair needs this. You can buy separate pillows especially shaped to provide this support; see any home medical supply or physical therapy source for better details.