Triangular surfaces may have internal curves only if the internal curves that are perpendicular to the natural boundary pass through the degenerate vertex. After a triangular surface has internal curves, you cannot change the natural boundary without first removing the internal curves.

If you try to change the natural boundary for a surface that has one or more internal curves, a message is displayed:

If you click Yes, the internal curves are deleted. If you click No, the natural boundary remains unchanged.

Alternatively, on the References tab, you can clear the check box next to the natural boundary and select the check box beside the required curve to designate the selected curve as the natural boundary.