  String theory requires two kinds of loop expansion: classical $(\alpha')$
worldsheet loops with expansion parameter $<T>$ where $T$ is a modulus field,
and quantum $(\hbar)$ spacetime loops with expansion parameter $<S>$ where $S$
is the dilaton field. Four-dimensional string/string duality (a corollary of
ten-dimensional string/fivebrane duality) interchanges the roles of $S$ and $T$
and hence interchanges classical and quantum.
