The theoretical description of contact angle 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 γ SG, the solid–liquid interfacial energy by γ SL, and the liquid–vapor interfacial energy (i.e. the surface tension) by γ LG, then the equilibrium contact angle θ C is determined from these quantities by the Young equation : γ γ S G − − γ γ S L − − γ γ L G cos ⁡ ⁡ θ θ C = 0