in order to make the technique more scientific rather than a mere interpretation by sight, what is called the Retention Value (Rf value for short) was applied in chromatography. A particular compound will travel the same distance along the stationary phase by a specific solvent (or solvent mixture) given that other experimental conditions are kept constant. In other words, every compound (dye, pigment, organic substance etc) have a specific Rf value for every specific solvent and solvent concentration. Rf values come very handy for identification because one can compare Rf values of the unknown sample (or its consituents) with Rf Values of known compounds.