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I am still confused in selecting whether to use \text or \tag for annotating equations. Which one should I use? If it is about personal preference, what is the pros and cons between them?

\documentclass[preview,border=12pt,12pt]{standalone}
\usepackage{mathtools}

\begin{document}

\section*{With \textbackslash text}
\begin{align*}
v = u + at && \text{without $s$}\\
s = vt - \tfrac{1}{2}at^2 && \text{without $u$}\\
s = ut + \tfrac{1}{2}at^2 && \text{without $v$}\\
s = \frac{v+u}{2}t && \text{without $a$}\\
v^2 = u^2 +2as && \text{without $t$}
\end{align*}


\section*{With \textbackslash tag}
\begin{gather}
v = u + at \tag*{without $s$}\\
s = vt - \tfrac{1}{2}at^2 \tag*{without $u$}\\
s = ut + \tfrac{1}{2}at^2 \tag*{without $v$}\\
s = \frac{v+u}{2}t \tag*{without $a$}\\
v^2 = u^2 +2as \tag*{without $t$}
\end{gather}
\end{document}

enter image description here

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4  
use \text, in my opinion comments does not belong in the area where we normally find equations numbers. This will only end up confusing readers. –  daleif Jun 27 at 9:42

1 Answer 1

up vote 13 down vote accepted

\tag is intended to be used a a one-off equation number as an alternative to just adding one to the last number used. In particular it becomes the default text for \ref. This gives some clues as to when to use one or the other. If the comment is essentially part of the equation as in your examples, use \text you will never want see equation \ref{foo} to make

see equation without t

On the other hand if the comment is a kind of label that you may want to reference, use \tag.

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