  A first order phase transition leading to deconfinement and chiral
restoration is a likely possibility for QCD, at least in some region of the
temperature-density plane. A signal for a unique transition is that the order
parameters for such transitions (which can be understood in terms of symmetries
only in limiting situations of very massive or massless quarks) are both
discontinuous at the same critical temperature. We show that such a situation
can be understood on a precise thermodynamical basis because of a general
relation among discontinuities which holds for first order transitions. We
derive the result by a generalization of the Clausius-Clapeyron equation and
also through the effective action formalism. We illustrate the theorem in an
elementary example.
