Abstract
The renormalisation group scheme of Migdal (1976) and Kadanoff (1976) is applied to a ferromagnetic Potts model on two hierarchical lattices: in the two cases the renormalisation transformation is split into two elementary processes (parallel bonds or series bonds) and each lattice can be mapped onto the other one. It is then easy to deduce the characteristic parameters (critical exponents, repulsive fixed points, etc) of one lattice from those of the other lattice. Moreover the classical geometrical duality offers a third possibility of mapping and the relationship between the renormalisation and the duality methods is presented in the peculiar case of a two-dimensional lattice.