The Ising model has a description as a random cluster model due to Fortuin and Kasteleyn. In this description, the natural observables are connectivities of clusters, i.e. probabilities that a number of points belong to the same cluster. The Ising model can then be viewed as the case q = 2 of the q -state Potts model, whose parameter q can vary continuously, and is related to the central charge of the Virasoro algebra.