two different compactifications (i.e. choices of vacua) of a given String Theory, then dualities can be seen as symmetries in the space of vacua. The extrapolation of this fact to the cases in which the theories are not known to originate from the same theory by different choices of vacuum is the basis of *M Theory*.

11. $\bigoplus{\rm\bf 
        Superstring~Theories} \stackrel{{\rm \bf 
          Witten~et~al.}}{\Longrightarrow}
      {\rm\bf  M~theory}$

    In this (super-) unification scheme, all the superstring theories are understood as different duality-related vacua of an unknown theory called M Theory, whose low energy limit is $N=1,d=11$ supergravity, which was discovered by Witten [@Witten:1995ex] to be related to the strong-coupling limit (S duality) of the low-energy limit of Type IIA Superstring ($N=2A,d=10$ Supergravity).

    Now we are back, in a sense, into the old Kaluza-Klein supergravity scenario, but all the Supergravity fields have got a String Theory meaning. It is amusing to see how, in this scheme, the low-energy limit of the Heterotic Superstring , which has chiral fermions, is related to the low-energy limit of M theory: 11-dimensional supergravity, which was apparently forbidden by Witten’s no-go theorem [@Witten:1983ux]. The solution to the inconsistency is the use of non-smooth manifolds (orbifolds) [@Horava:1995qa], evading one the hypothesis 