In the context of planar chromatography, Rf is the ratio of the distance traveled by the center of a spot to the distance traveled by the solvent front. Suppose a particular substance in an unknown mixture travels 2.5 cm and the solvent front travels 5.0 cm, the retention factor would be 0.5. An Rf value will always be in the range 0 to 1 and not greater than 1; if the substance moves. The substance can only move in the direction of the solvent flow, and cannot move faster than the solvent.