TeX/ifdim

Synopsis
\ifdim   [\else ] \fi

Description
The \ifdim command denotes the start of an if-then-else control structure. The forms  and  must expand to dimensions, while  must be one of the characters '=', '<', or '>'. If  expands to a true expression then  is processed; otherwise it is ignored. If the \else section is included and  expands to a false expression, then  is processed; otherwise it is ignored.