What is in contact with chair if you have little fat or muscle on your tush?

Iscial tuberosities. The "sit bones" are part of the pelvis called the ischial tuberosities. The hamstring muscles also attach to these bones and there is a "bursa" which is a sac of fluid which protects the tendon of the muscles from rubbing against the bone. Sometimes these bursa get irritated and can cause pain where you sit. Also this is a common place for hamstring tears. Many treatments available to help.