Existing posts regarding tagging for aligned I've found are all about numbering/referencing, but that's not my concern here.

I often would like to add remarks or short phrases at the end or beginning of equations in the aligned environment (inside a bigger math environment), like saying "substitute $y = f(x)$", "or", "denote $u \equiv 1/x$", etc.

Individual lines in aligned doesn't allow \tag. Most of the time I just use \qquad \text{my remarks} at the end. Sometimes I can get by with the optional placement [t],[c],[b] to fit the \tag for the whole aligned to the desired line.

What would be a good practice to do this? Or is it just a habit I should get rid of, commenting along with the equations when using aligned?

Thank you.

P.S. MWE is not needed for this...I think?

closed as primarily opinion-based by Martin Schröder, Zarko, Maarten Dhondt, Andrew Swann, Mensch Dec 31 '16 at 13:25

Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise. If this question can be reworded to fit the rules in the help center, please edit the question.

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    You always provide a mwe as common courtesy. Without seeing an example, I would probably never include that sort of remarks inside the displayed math. Always outside for clarity. But again there can be exceptions – daleif Dec 25 '16 at 8:32
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    Yes, a MWE is needed. At least an approximation of what you'd like to get. – egreg Dec 25 '16 at 9:49
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    See tex.stackexchange.com/q/27283/15925 - particularly the solution with the most votes using align, rather than the accepted solution. – Andrew Swann Dec 31 '16 at 13:19

If the explanations can be written in the body text, that is usually to be preferred. In certain cases, however, small remarks can be beneficial. I have used it when teaching linear equations as shown in the image below - written in Danish. Describing the steps of solving equations

If this is the effect you want, add another alignment character in your align-environment.

    3(2+x)  &=x+8       &\\
    6+3x    &=x+8       &\text{(Gang parentesen ud)}\\
    6+3x-x  &=8         &\text{(Saml $x$'erne på samme side)}\\
    2x      &=8-6       &\text{(Isolér $x$'erne)}\\
    2x      &=-2        &\\
    x       &=\frac{-2}{2}  &\text{(Divider med koefficienten foran $x$)}\\
    x       &=-1.

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