In the saturated mixture region the [[Substance|substance]] is partially [[Liquid|liquid]] and partially [[Gas|gaseous]]. This region lies between the saturated liquid and saturated vapour lines, $\nu_f$ and $\nu_g$​ at a given [[Pressure|pressure]]/[[Temperature|temperature]]. Quality, $x$, determines how much of the substance is liquid or gas. Quality is defined as the ratio of gaseous [[Mass|mass]] to the total mass of the substance: $x=\dfrac{m_g}{m}$ The quality of the substance can be expressed in terms of [[Specific Volume and Density|specific volume]]: $\nu=(1−x)\nu_f+x\nu_g​$ $\nu=x\nu_{fg}+\nu_f​$