Normals are imaginary lines perpendicular to each point on a curve or surface.

The direction of U and V isoparms on a surface determines the direction of the surface’s normals.

Normals are a mathematical side-effect of NURBS.They are often used as a way of specifying which side of a surface points “inside” or “outside” (for example, when creating shells).

Normals are also an indirect indicator of the shape of a curve or surface. Since they are always perpendicular to the curve or surface, the way normal lines point toward or away from each other can reveal subtle curvature.