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I try to make a comment on the right (or left) of one line of proof. But the align* environment automatically move the whole equations to the left to leave space for the comment. But I still want the equations to be in the center (the comment is short enough that it won't be conflicting the equations). How could I achieve this?

To be more specific, I used the code

 equation0 &= equation1
           &= equation2
           &= equation3 && \text{make a comment here}
           &= equation4

I just want to put \text{make a comment here} on the right without affecting the original alignment of the equations.

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up vote 13 down vote accepted

Omit the && and use \rlap{\text{make a comment here}}, which causes the comment to have no width "to the right", meaning it starts at the same place but doesn't actually take up any space in the alignment. For a comment on the left, use \llap instead of \rlap, which causes leftward space to be ignored.

In response to the question in the comment: in align* or equation* specifically (or any other unnumbered environment) you can hijack the "equation label" to contain a comment by using \tag*{\text{make a comment here}} rather than \rlap. This will place the comment on the left if amsmath was loaded with the leqno option. In a numbered equation, it is not a good idea to put something flush right.

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It works perfectly! Thank you! – user69818 Apr 14 '13 at 1:19
By the way, do you know how to make the "\rlap{\text{make a comment here}}" thing to the rightmost? It seems that \hfill or \raggedright does not work. – user69818 Apr 14 '13 at 3:01
Added to the answer. – Ryan Reich Apr 14 '13 at 3:05
Thanks for the answer. But before today I always used amsart and \tag to make comments on equations (by default of amsart, \tag appears on the left). But this time the comment is a bit longer that makes the equations move to the right. – user69818 Apr 14 '13 at 3:12
Instead of \rlap{...} I would recommend \makebox[0pt][l]{...} that's slightly less efficient but also less surprising in other contexts (try starting a paragraph with \rlap). – egreg Apr 14 '13 at 8:37

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