About Triangular Surfaces
Triangular surfaces in Style are created in the same way as rectangular surfaces, but they have three boundaries instead of four. Triangular surfaces have one degenerate edge. The edge opposite the degenerate vertex is called the natural boundary. When you create a triangular surface, the first boundary curve that you select is the natural boundary.
Surface operations that are valid for rectangular surfaces are also valid for triangular surfaces, including the following options:
Edit Definition
You can change a triangular surface into a rectangular one by using Edit Definition to add another boundary curve.