  We discuss the possible origin of the duality observed in the quantum Hall
current-voltage characteristics. We clarify the difference between
"particle-vortex" (complex modular) duality, which acts on the full transport
tensor, and "charge-flux" ("real") duality, which acts directly on the filling
factor. Comparison with experiment strongly favors the form of duality which
descends from the modular symmetry group acting holomorphically on the
compexified conductivity.
