A term which is equivalent to relative retention used in column chromatography: the ratio of the value of a component to the value of a standard (reference) substance. Since the mobile phase front is common for the two components, the value can be expressed directly as the ratio of the distances travelled by the spot of the compound of interest () and the reference substance () respectively: In former nomenclatures the symbol was used to express relative retardation in planar chromatography. Because of its identity with the symbol for peak resolution the symbol is suggested for relative retardation in planar chromatography.