  Duality relations are obtained for correlation functions of the q-state Potts
model on any planar lattice or graph using a simple graphical analysis. For the
two-point correlation we show that the correlation length is precisely the
surface tension of the dual model, generalizing a result known to hold for the
Ising model. For the three-point correlation an explicit expression is obtained
relating the correlation function to ratios of dual partition functions under
fixed boundary conditions.
