Water saturation  is the fraction of porevolume that is filled with water, and similar for oil saturation . Thus, saturations are in itselves scaled properties or variables. This gives the constraint
The model functions or correlations for relative permeabilities in an oil-water system are therefore usually written as functions of only water saturation, and this makes it natural to select water saturation as the horizontal axis in graphical presentations. Let  (also denoted  and sometimes ) be the irreducible (or minimal or connate) water saturation, and let  be the residual (minimal) oil saturation after water flooding (imbibition). The flowing water saturation window in a water invasion / injection / imbibition process is bounded by a minimum value  and a maximum value . In mathematical terms the flowing saturation window is written as
By scaling the water saturation to the flowing saturation window, we get a (new or another) normalized water saturation value
and a normalized oil saturation value