Our 'Sitting Bones' are more formally called the ischial tuberosities. These tuberosities are two curved horseshoe-shaped bones, which extend from the ischium in the base of the pelvis.
The weight of our body balances on our sitting bones, which also act like rockers, thus allowing some pelvic movement whilst still sitting. If you wish to know where they are, sit in a poised position on a firm surface, with your hip joints free, and then rock from side to side, or back to front. The two knobbly bones you can feel as your weight transfers from buttock to buttock, are your sitting bones.