Geometrical proof that L1 incidence represents PI/LL mismatch. In a triangle of ABC, consisting of a line connecting the center of the femoral heads and the center of the sacral upper endplate, an imaginary vertical line, and L1 upper endplate, the angle α is equal to the sum of PT and (90 – L1S) and α is calculated as (90 + PI – LL). Given that L1I is the complementary angle of α, L1I is equal to (PI – LL). L1I = L1 incidence, L1S = L1 slope, LL = lumbar lordosis, PI = pelvic incidence.