  Quark-meson duality for two-point functions of vector and axial-vector QCD
currents is investigated in the large-Nc approximation. We find that the joint
constraints of duality and chiral symmetry imply degeneracy of excited vector
and axial-vector mesons in the large-Nc limit. We compare model-independent
constraints with expectations based on the Veneziano-Lovelace-Shapiro string
model. Several models of duality are constructed, and phenomenological
implications are discussed.
