A detailed study of the effects of forward flexion in the spine on back shape is reported. ISIS and the Scoliometer were used to record angle of trunk inclinations (ATIs) in 13 patients with idiopathic scoliosis. Two standard positions were used: standing erect (for ISIS) and sitting forward bending (for the Scoliometer). In the lumbar region, a positional change in ATI between standing erect and sitting forward is revealed; it is related to the type and side of mainly compensatory spinal curves. It does not correlate with Cobb angle but it does correlate with each of spinous process rotation (Bunnell) and pedicular rotation (Perdriolle). In the thoracic region, the change from a standing-erect to a sitting-forward-bending position, in contrast, shows a statistically significant reduction of the hump on the convexity of the spinal curve. The findings have relevance to screening tests for scoliosis.