Example

Consider a generatrix that follows the contour of a simple closed line called the directrix. This generatrix generates a surface. Depending on the shape of the simple closed line, the generatrix, fixed at apex A :

• generates a conical surface, if the directrix is a closed curve;

• generates a pyramidal surface, if the directrix is a polygon.

Depending on the shape of the simple closed line, the generatrix, while always maintaining the same direction :

• generates a cylindrical surface, if the directrix is a closed curved;

• generates a prismatic surface, if the directrix is a polygon.