The tendency of a given chemical species to partition itself
preferentially between liquid and vapor phases is the Henry's law constant. There can be VLE data for mixtures of four or more components, but such a boiling-point diagram is hard to show in either tabular or graphical form. For such multi-component mixtures, as well as binary 
mixtures, the vapor–liquid equilibrium data are represented in terms of  values (vapor–liquid distribution ratios) defined by
where  and  are the mole fractions of component  in the phases  and  respectively.
For Raoult's law   
For modified Raoult's law
where  is the activity coefficient,  is the partial pressure and  is the pressure.
The values of the ratio  are correlated empirically or theoretically in terms of temperature, pressure and phase compositions in the form of equations, tables or graph such as the DePriester charts.
For binary mixtures, the ratio of the  values for the two components is called the relative volatility denoted by 
which is a measure of the relative ease or difficulty of separating the two components. Large-scale industrial distillation is rarely undertaken if the relative volatility is less than 1.05 with the volatile component being  and the less volatile component being .
 values are widely used in the design calculations of continuous distillation columns for distilling multicomponent mixtures.