Each face in a mesh has a perpendicular normal vector whose direction is determined by the order in which the vertices are defined and by whether the coordinate system is right- or left-handed. If the normal vector of a face is oriented toward the viewer, that side of the face is its front. In Direct3D, only the front side of a face is visible, and a front face is one in which vertices are defined in clockwise order.
Direct3D applications do not need to specify face normals; the system calculates them automatically when they are needed. The system uses face normals in the flat shade mode. For Gouraud shade modes, and for controlling lighting and texturing effects, the system uses vertex normals.
Throughout Direct3D, vertices describe position and orientation. Each vertex in a primitive is described by a vector that gives its position and a normal vector that gives its orientation, texture coordinates, and a color.