A letter followed by a 2 (occasionally a superscript 2) denotes two turns, or a 180 degree turn.

Before it is registered, it is common practice (with some legal standing) to use the service mark symbol (a superscript SM).

Isotopes Although isotopes are more relevant to nuclear chemistry or stable isotope chemistry than to conventional chemistry, different isotopes may be indicated with a prefixed superscript in a chemical formula.