The erector spinae is attached to the medial crest of the sacrum (a slightly raised feature of the sacrum closer towards the midline of the body as opposed to the "lateral" crest which is further away from the midline of the body), to the spinous processes of the lumbar (bony points along your lower back) and the eleventh and twelfth thoracic vertebrae and the supraspinous ligament, to the back part of the inner lip of the iliac crests (the top border of your hips), and to the lateral crests of the sacrum, where it blends with the sacrotuberous and posterior sacroiliac ligaments.