Inside a liquid-vapor coexistence region, part of a pure substance is saturated liquid and the rest is saturated vapor.

The vapor quality $x$ is the fraction of the total mass that is vapor:

$$x=\frac{m_g}{m_f+m_g},\qquad 0\le x\le1.$$

Here $f$ denotes saturated liquid and $g$ saturated vapor. Quality is defined only inside the two-phase mixture region.

For any known specific property $y$ whose corresponding extensive quantity adds across the two phases, the mixture value is

$$y=y_f+x(y_g-y_f)=y_f+xy_{fg}.$$

For example, if saturated water at a given pressure has $v_f=0.001\ \mathrm{m^3/kg}$ and $v_g=1.20\ \mathrm{m^3/kg}$, then a mixture with $x=0.25$ has

$$v=0.001+0.25(1.20-0.001)\approx0.301\ \mathrm{m^3/kg}.$$

Although only one quarter of the mass is vapor, the vapor occupies almost all the volume because $v_g\gg v_f$. The same mixture rule can later be applied to other additive state properties once they are known.