For a single substance the definition of the molar Gibbs free energy can be written in the form f = g − − p v . Thus when p and g are constant along with temperature the function f ( T R , v ) represents a straight line with slope − − p , and intercept g . Since the curve, f ( T R , v ) , has positive curvature everywhere when T R ≥ ≥ T c , the curve and the straight line will be have a single tangent. However, for a subcritical, T R , with p = p s ( T R ) and a suitable value of g the line will be tangent to f ( T R , v ) at the molar volume of each coexisting phase, saturated liquid, v f ( T R ) , and saturated vapor, v g ( T R ) ; there will be a double tangent. Furthermore, each of these points is characterized by the same value of g as well as the same values of p and T R . These are the same three specifications for coexistence that were used previously.