the sacrum or bottom tip of backbone.
pelvis where sacrum is suspended in.
Not sure about the square
"The sacrum suspends as a compression element within the musculo-ligamentous envelope and transfers its loads through that tension network. Even when a person stands on one leg, the sacrum sits within its tension network. This tension network provides omni directional structural stability, independent of gravity and hierarchical. The rim could distribute its load, rather than locally loading the forces at a point."