\ifvmode <TeX code 1> [\else <TeX code 2>] \fi
The \ifvmode command denotes the start of an if-then-else control structure that tests whether TeX is currently in either vertical mode or internal vertical mode. As indicated in the Synopsis, the \else part is optional. <TeX code 1> is processed if TeX is in one of the vertical modes, otherwise it is ignored. If the \else section is included and TeX is not in one of the vertical modes then <TeX code 2> is processed; otherwise it is ignored.