Residual properties should not be confused with excess properties, which are defined as the deviation of a thermodynamic property from some reference system, that is typically not an ideal gas system. Whereas excess properties and excess models (also known as activity coefficient models) typically concern themselves with strictly liquid-phase systems, such as smelts, polymer blends or electrolytes, residual properties are intimately linked to equations of state which are commonly used to model systems in which vapour-liquid equilibria are prevalent, or systems where both gases and liquids are of interest. For some applications, activity coefficient models and equations of state are combined in what are known as " γ γ - ϕ ϕ models" (read: Gamma-Phi) referring to the symbols commonly used to denote activity coefficients and fugacities.