An oligomeric branch may be termed a short-chain branch, and a polymeric branch may be termed a long-chain branch. Side chains have noteworthy influence on a polymer's properties, mainly its crystallinity and density.

In polymer science, the side chain of an oligomeric or polymeric offshoot extends from the backbone chain of a polymer. The R symbol was introduced by 19th-century French chemist Charles Frédéric Gerhardt, who advocated its adoption on the grounds that it would be widely recognizable and intelligible given its correspondence in multiple European languages to the initial letter of 'root' or 'residue': French racine ('root') and résidu ('residue'), these terms' respective English translations along with radical (itself derived from Latin radix below), Latin radix ('root') and residuum ('residue'), and German Rest ('remnant' and, in the context of chemistry, both 'residue' and 'radical'). To indicate other non-carbon groups in structure diagrams, X, Y, or Z are often used.

The placeholder R is often used as a generic placeholder for alkyl (saturated hydrocarbon) group side chains in chemical structure diagrams.