The pitch of a closed developable surface is equaî to the arclengzh of its edge of regression. The aim of this paper is to show that this is characteristic property of a closed developable. viz. that if the pitch of a closed ruled surface is equal to the are- length of its line of striction then the surface is developable. 1. The Pitch [1] of a closed [2] ruled surface R (i. e. a ruled surface whose directrİK curve D and spherical indicatrix of the generators J are both simply connected closed curveş) is defined as the line integral