A line (as of cord) that has at one end a weight (as a plumb bob) and is used especially to determine verticality.
The Scoliosis research society (SRS) has previously described
1)
normal sagittal balance as occurring when a plumb line drawn from the centre of the body of C7 lies within ±2 cm of the sacral promontory.
1)
Scoliosis Research Society. White paper on sagittal plane alignment