The fourth lumbar vertebra
The fourth lumbar vertebra is a reference point of the bone structure to take body measurements.
The fourth lumbar vertebra is located at the most pronounced point of the lumbar lordosis, which is the curve we observe in the lower back.
Above this vertebra is located half of the back waist seam of the lower garments.
More information:
Anatomy of the human body by Gray, Henry, 1825-1861; Lewis, Warren Harmon, 1870
Go to “Measurement reference points”