An open or closed element with vertices or endpoints.

For a profile consisting of lines and arcs, the elements may or may not be tangent at their endpoints.

For a closed b-spline curve, a closed curve that is not tangent where its start and end point meet. Open b-spline curves are always non-periodic.

Because non-periodic elements have vertices or endpoints, they have a physical start point that can be selected when constructing lofted or swept surfaces.