Purpose of DOTSB in definition of \iff

What exactly is the purpose of the \DTOSB in the definition of \iff? There is a mention in Is there a left-handed equivalent of \implies? that the \DOTSB makes \iff "dots aware". But, when I try to use \dots on either side I don't see any difference.


\newcommand{\MyIffWithDotsbNonStar}{\DOTSB\;\Longleftrightarrow \;}


the produces the same meaning as \iff.

MWE:

\documentclass{article}
\usepackage{mathtools}
\usepackage{xcolor}
\usepackage[paperwidth=10in]{geometry}% for better display of output

\newcommand{\MyIffWithDotsbNonStar}{\DOTSB\;\Longleftrightarrow \;}
\newcommand{\MyIffWithoutDotsbNonStar}{\;\Longleftrightarrow \;}

\newcommand*{\MyIffWithDotsbStar}{\DOTSB\;\Longleftrightarrow \;}
\newcommand*{\MyIffWithoutDotsbStar}{\;\Longleftrightarrow \;}

\begin{document}
\par \verb|\iff|: \texttt{\meaning\iff}
\par \verb|\MyIffWithDotsbNonStar|: \texttt{\meaning\MyIffWithDotsbNonStar}
\par \verb|\MyIffWithDotsbStar|: \texttt{\meaning\MyIffWithDotsbStar}

\medskip
\par $a \iff b$ using \verb|\iff|
\par $a \MyIffWithDotsbNonStar b$ using \verb|\MyIffWithDotsbNonStar|
\par $a \MyIffWithoutDotsbNonStar b$ using \verb|\MyIffWithoutDotsbNonStar|
\par $a \MyIffWithDotsbStar b$ using \verb|\MyIffWithDotsbStar|
\par $a \MyIffWithoutDotsbStar b$ using \verb|\MyIffWithoutDotsbStar|

\medskip
\par $a \dots \iff \dots b$ using \verb|\dots \iff \dots|
\par $a \dots \MyIffWithDotsbNonStar \dots b$ using \verb|\dots \MyIffWithDotsbNonStar \dots|
\par $a \dots \MyIffWithoutDotsbNonStar \dots b$ using \verb|\dots \MyIffWithoutDotsbNonStar \dots|
\par $a \dots \MyIffWithDotsbStar \dots b$ using \verb|\dots \MyIffWithDotsbStar \dots| \textcolor{red}{\bfseries DIFFERS}
\par $a \dots \MyIffWithoutDotsbStar \dots b$ using \verb|\dots \MyIffWithoutDotsbStar \dots|

\end{document}

• You need \newcommand* in order \DOTSB to have an effect, see tex.stackexchange.com/a/266721/4427 – egreg Jan 2 '16 at 9:38
• @egreg: It seems that \meaning\iff and \meaning\MyIffWithDotsbNonStar (defined with \newcommand (i.e. without the star option) gives the same meaning. – Peter Grill Jan 2 '16 at 10:08
• Yes, \iff has a wrong definition in amsmath.sty – egreg Jan 2 '16 at 10:13
• I'd like to know as well. Was editing a set of notes and was using a variant of iff where I'd added a quad on both sides. You can see the alignment difference if you add it into an align – daleif Jan 2 '16 at 10:15

• Does this ignore \protected too? – Manuel Mar 4 '16 at 10:14
• Okey, I thought it would be easier to gobble everything until -> and look for \DOTSB. – Manuel Mar 4 '16 at 10:21