Compounds of helical curves: Medieval twisted columns

Twisted columns are ubiquitous in cult places, but classical treatises discuss only some specific geometric models. We address medieval columns, which elude this description, and introduce a parametric model mathematically describing a great variety of columns, and their differences. The model is validated through 3D survey. We describe the variety of columns of the Cloister of Saint Paul outside the walls,(A.D.1204-45), in Rome. When a column looks more wrought or torqued, it is obtained by compounding several different helices.