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UVS surface with the stable (blue), metastable (yellow), and unstable (red) regions differentiated. Following Gibbs, the upper transparent plane rolls in a single direction to mark the blue-yellow boundary of saturated, two-phase states. The lower plane rolls with two degrees of freedom to show single-phase (vapor) properties. Chips intersecting the black-and-white checkered axis through the origin are extensions of the planes and show values of the chemical potential corresponding to the tangency points of both.