Some familiar shapes in three-dimensional geometry include the sphere and surfaces of revolution—among them the torus, that doughnut shaped like a lifebuoy or inner tube. For this surface, the generating circle viewed from above traces an annulus.
Now take a generating circle whose center still traces a circle, but which remains parallel to itself in a plane perpendicular to the circle of centers. Viewed from above, the successive positions of the generating circle and the resulting surface now look like this:
