General considerations concerning the number of virtual phases

A formula is deduced showing that ρ phases becoming critically identical should be counted as 2ρ-1 virtual phases. Again no more than two phases having the same composition can be in equilibrium, whatever the number of components. Finally two phases having the same composition cannot be in equilibrium with a third phase with which one of these phases becomes critically identical. A few examples from the literature found to be at variance with these rules are discussed.