2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/149150String theory requires two kinds of loop expansion: classical $(α')$ 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.5 pages latex, NI-94-016, Typographical errors correctedHigh Energy Physics - TheoryHigh Energy Physics - Phenomenologytext