Large-Small Equivalence in String Theory
  The simplest toroidally compactified string theories exhibit a duality between large and small radii: compactification on a circle, for example, is invariant under R goes to 1/R. Compactification on more general Lorentzian lattices (i.e. toroidal compactification in the presence of background metric, antisymmetric tensor, and gauge fields) yields theories for which large-small invariance is not so simple. Here an equivalence is demonstrated between large and small geometries for all toroidal compactifications. By repeatedly transforming the momentum mode corresponding to the smallest winding length to another mode on the lattice, it is possible to increase the volume to exceed a finite lower bound. 
Eva Silverstein