From Scoliosis Wiki

The axis-line distance technique (ALDT) scoliosis measurement works as follows: A line connecting the midpoint of the superior border of the pubic symphysis and the central point of the seventh cervical vertebra is denoted the axis line, and it is used as a baseline for measurement. The distance between the axis line and the center of the apex vertebra is used to assess the degree of scoliotic curvature. To determine the apex vertebra, another line parallel to the axis line is drawn along the convex side of the vertebra at the peak of a scoliotic curve, and this outermost vertebra is considered the apex vertebra.

For people without scoliosis, the center of the vertebrae in a normal x-ray image should be in alignment with the axis line.