Webster: wedge shaped piece at crown of arch that locks the other pieces in place, something on which associated things depend for support.

Lexicon: a center stone in an arch, bearing lateral and vertical
stress. Note: sacrum identified as center stone; biped weight
bearing legs document sides.

Keystone sacrum engineered design by physics. Identified by triangular
wedge shaped, locking horizon base with lateral vertical sacro-iliac
semi - lunar articular biped junctions.