A closed B-spline starts and ends at the same point, and encloses an area.

A closed B-spline can also be periodic, which means that all derivatives of the curve (less than order -1) are continuous through the points. In other words, a periodic B-spline passes "smoothly" through the point at which its ends are joined, without a kink in the curve.

In a design, a B-spline whose ends do not meet can be called "open." Mathematically, however, an open B-spline starts at its first pole and ends at its last pole, and the ends need not meet. Setting the Closure tool setting to Open results in mathematically open B-spline curves.

You can use the to change a periodic B-spline's definition in the model to be a mathematically open B-spline without changing its shape. This is helpful when the DGN file is to be transferred to a package that does not support periodic B-splines.