A lumbosacral trapezium can be constructed using the parallels to the lower plate L4 and the upper plate S1 and two perpendiculars to the former; it yields exact measurement data for changes in height and curvature in the lower lumbar motion segments. The clinically relevant reductions in height in the dorsal part of the segments L4-5 and L5-S1, the increase in lumbar lordosis, and therapeutic straightening by means of conservative or operative treatment can thus be plotted exactly.