For a polygon the surface normal can be calculated as the vector cross product of two edges of the polygon.

In 3-D computer graphics, triangles are often used as the basic "building blocks" of a polygon since triangles are guaranteed to be planar. That is, given three points in space (*x*, *y*, *z*), the smallest surface connecting them all is guaranteed to be a flat plane. The same is not necessarily true for more complex polygons, such as quadrilaterals. A quad *can* be a flat plane, but given any four points, there is no guarantee that a flat plane connecting all four points can be generated. Since only one normal can exist for any given surface, bent surfaces (such as on a sphere) have to be broken up into component triangles.

*See also:* computer graphics

- An explanation of normal vectors from Microsoft's MSDN