This paper proposes a simple parametric system to generate an almost complete set of ruled surfaces that may be used to describe building geometry. The major classes of regular, named ruled surfaces can be generated from a limited set of curves. Each of these is shown to be reducible to a transformation of a single standard curve, a helix, and therefore represented by a limited set of six parameters. Six extra parameters can position each surface on a global coordinate system. The representation is designed to be flexible enough to represent both planar and curvilinear forms, producing a description of form from minimal data.