Sinusoidal functions provide a suitable mathematical representation of the frontal projection of the scoliotic spine. This is due to their obvious similarity with the spinal curve. Three parameters (amplitude, wavelength and phase) are sufficient for a complete description of the curve. Unfortunately the sinusoidal approach fails in asymmetric curves, e.g. double curves with distinctly different curvatures. In this paper a new approach is presented, which is based on modulated sine curves, leaving 4 parameters to be adjusted. The fit of these functions to radiographic curves (478 cases) is described. The relevance of this approach in reconstructing the spine from back surface measurements is pointed out.