Chiral Lagrangians contain in their coefficients information concerning the underlying fundamental theory. We show that to a large measure this structure is determined by a duality with the low-mass spectrum of the theory. This is tested in some model theories and then applied to QCD. In the real world, only the vector mesons are light enough to pass our criteria for being low mass, and in practice they do seem to dominate the structure of the phenomenological Lagrangians. This raises the possibility of a fusion between vector-dominance ideas and rigorous chiral Lagrangian methods.