In a structural diagram or formula, the letters R or X are used to represent a substituent group. An organic substituent such as methyl, ethyl, aryl can be written as R (or R, R, etc.) This is a generic placeholder. The "R" derived from "radical" or "rest". The R can represent as many atoms as the author wants. The first to use this symbol was Charles Frédéric Gerhardt in 1844.