The literature has not many interesting examples of ruled surfaces in Euclidean space besides cylinders, cones, hyperboloids, and the helicoid. Let’s fix that. A *cone*, or more precisely a *frustum* (latin for *piece*), can be described by following a horizontal circle (as a directrix) counterclockwise and rotating the generators also counterclockwise, horizontally pointing in the same direction as the point on the directrix, and with a fixed vertical component.

The result is a *flat* surface and thus not interesting for making a *curved* book. But we can also follow the circle and let the generators rotate the other way. I will call the resulting surface an *anti-cone*. It is certainly not flat anymore.

Take for a ruled surface all of its generators (the straight lines) and shift them so that they pass through the origin.

Their intersections with a sphere centered at the origin is called the *spherical indicatrix* of the ruled surface.

In this case, the spherical indicatrix is a pair of horizontal circles, both for cone and anti-cone, but differently orientated.

An old theorem about ruled surfaces states that you can deform any ruled surface by changing its spherical indicatrix pretty much arbitrarily. It turns out that there are typically two different solutions to do this, even if we trace the indicatrix in the same direction.

In other words, for a given ruled surface, there is a second ruled surface along a different directrix but with pairwise parallel generators. Let’s call this the *doppelgänger* of the ruled surface. For the anti-cone, it looks like this:

One can visualize the relationship between the two surfaces by putting them together and coloring corresponding (i.e. parallel) lines with the same color.

If we had paper ribbons shaped this way, we could bend one into the other.

Here is another image of the generators of the anti-cone doppelgänger. Giving up on clarity can increase the esthetical appeal when we add ambiguity.