A term which is equivalent to relative retention used in column chromatography: the ratio of the $R_{\text{F}}$ value of a component to the $R_{\text{F}}$ value of a standard (reference) substance. Since the mobile phase front is common for the two components, the $R_{\text{F}}$ value can be expressed directly as the ratio of the distances travelled by the spot of the compound of interest ($b_i$) and the reference substance ($b_{\text{st}}$) respectively: $$R_{\text{rel}}=\frac{R_{\text{F}\left(i\right)}}{R_{\text{F}\left(\text{st}\right)}}=\frac{b_i}{b_{\text{st}}}$$ In former nomenclatures the symbol $R_{\text{s}}$ was used to express relative retardation in planar chromatography. Because of its identity with the symbol for peak resolution the symbol $R_{\text{rel}}$ is suggested for relative retardation in planar chromatography.