The basis for the rule (Atkins and de Paula, justification 6.1) is that equilibrium between phases places a constraint on the intensive variables. More rigorously, since the phases are in thermodynamic equilibrium with each other, the chemical potentials of the phases must be equal. The number of equality relationships determines the number of degrees of freedom. For example, if the chemical potentials of a liquid and of its vapour depend on temperature ("T") and pressure ("p"), the equality of chemical potentials will mean that each of those variables will be dependent on the other. Mathematically, the equation , where "μ" = chemical potential, defines temperature as a function of pressure or vice versa. (Caution: do not confuse "p" = pressure with "P" = number of phases.)