Back view of pelvis, sacrum, and two lumbar vertebrae. Sacroiliac joint is where sacrum meets pelvis. Sacrum is lowest part of spinal column. Ligaments hold sacrum and pelvis together.

Posterior view of pelvis and sacrum and ligaments on right side SOURCE: Original KOD art MOD: Added nerve roots (DWu)
References: Thieme Atlas of Anatomy General Anatomy and Musculoskeletal System
Natural bone skeleton