Sacral slope (SS) is angle subtended by horizontal reference line (HRL) and sacral endplate line (bc). SS shares common reference line (bc) with pelvic incidence (PI) and pelvic tilt (PT). PI is measured from static anatomic structures. PT and SS depend on angular position of sacrum/pelvis in relation to femoral heads, which changes with standing, sitting, and lying down. Relationship of PT and SS is affected by lumbosacropelvic flexion and extension. VRL, Vertical reference line.