The Gibbs Phase Rule indicates that for a two phase, single component thermodynamic system we will have one independent intensive parameter. Given that the Degree of Freedom is $1$ means that fixing one intensive parameter would fix the entire state of the system.
Any property $x$ is just a function of one other property say $y$, i.e. $x=f(y)$. This gives for two phase system $P=f(T)$, but why isn't $v=f(T)$?
As we know $v$ can have a value anywhere between $v_f$ and $v_g$. Are there any assumptions for the Phase Rule, other than equilibrium, which would explain this?