Line whose movement along a simple curve, called a directrix, creates a surface.
- Generatrix of a cone
Line that passes through a fixed point A and that moves in space following a simple closed curved line called the directrix of the cone. The surface created in this way is called the conical surface.
- Generatrix of a cylinder
Line that moves in space along a simple closed curved line in a constant direction. The surface created in this way is called the cylindrical surface.
- Generatrix of a prism
Line that moves in space along a closed polygonal chain in a constant direction. The surface created in this way is called the prismatic surface.
- Generatrix of a pyramid
Line that passes through a fixed point A and that moves in space following a closed polygonal chain called the directrix of the pyramid. This surface is called the pyramidal surface.