10d with the two supercharges having opposite chirality (type IIA) and one has $N=2$ supersymmetry in 10d with the two supersymmetries having the same chirality (type IIB). One of the theories is based on unoriented open and closed strings (type I) and the other four are based on oriented closed strings. In short, each of the five theories appears to be quite different from the others, with very distinctive features. Of course, we don’t really want five theories, since there is only one universe to explain. So the hope that I and others expressed in the mid 1980’s was that some of these might turn out to be equivalent or inconsistent, but it wasn’t apparent how this could happen.

In the second superstring revolution (1994–??) we are learning that all of these different superstring theories are consistent, but that they are non-perturbatively equivalent. Each of them represents a perturbation expansion of a single underlying theory about a distinct point in the moduli space of quantum vacua. Moreover, there is a sixth rather special point in this moduli space characterized by an 11d Poincaré-invariant vacuum. The rules for doing quantum mechanics in the 11d vacuum are not yet understood, but the 