In a chemical structural formula, an organic substituent such as methyl, ethyl, or aryl can be written as R (or R, R, etc.) It is a generic placeholder, the R derived from radical or rest, which may replace any portion of the formula as the author finds convenient. The first to use this symbol was Charles Frédéric Gerhardt in 1844.
The symbol X is often used to denote electronegative substituents such as the halides.