, two-sided lighting is specified. In this case,
                    vertices of back-facing polygons have their normals reversed
                    before the lighting equation is evaluated. Vertices of
                    front-facing polygons are always lighted with no change to
                    their normals. The initial value is 0. Note that there is
                    only one set of material properties shared by both
                    front- and back-facing polygons.</p></dd></dl></div><p>The 