It can be realized as a ruled surface. For example, any rectangle can be glued left-edge to right-edge with a reversal of orientation. Some, but not all, of these can be smoothly modeled as surfaces in Euclidean space. Such paper models are developable surfaces having zero Gaussian curvature , and can be described by differential-algebraic equations. A line drawn along the edge travels in a full circle to a point opposite the starting point.