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