The theoretical description of contact arises from the consideration of a thermodynamic equilibrium between the three phases: the liquid phase (L), the solid phase (S), and the gas or vapor phase (G) (which could be a mixture of ambient atmosphere and an equilibrium concentration of the liquid vapor). (The "gaseous" phase could be replaced by another immiscible liquid phase.) If the solid–vapor interfacial energy is denoted by formula_1, the solid–liquid interfacial energy by formula_2, and the liquid–vapor interfacial energy (i.e. the surface tension) by formula_3, then the equilibrium contact angle formula_4 is determined from these quantities by the Young equation: