The Pelvis once in position is a weight bearing bone in sitting. Just below the pubic arch is this picture are two semi circular bones, your sit bones, two beautiful ischial tuberosities.
These fabulous structures are the main stay of great posture in sitting. When these bones make contact with a hard surface, your seat for example or the floor, the pelvis is upright, it is impossible to ‘slouch’ from this position. I think about the spine as a tree truck, the pelvis as its root, and if the root of a tree are well ground down the tree can stand tall.
Wild Atlantic Physio 