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There are plenty of resources out there explaining how to align multiple lines of equations at the equals sign. But I can’t find anything that says when this should be done. Does any style guide, ISO standard, or similar publication address this?

Use cases I’ve seen include:

  1. One equation spanning multiple lines, e.g. $$ \begin{align} 0 &= x^2 - 4x + 4 \\ &= (x - 2)^2 \end{align} $$

  2. The next line is derived from the previous one, e.g. $$ \begin{align} 0 &= (x - 2)^2 \\ \therefore x &= 2 \end{align} $$

  3. The equations are a list of formulas, identities, etc. given together, e.g. $$ \begin{align} d &= 2r \\ C &= 2\pi r \\ \mathrm{Area} &= \pi r^2 \end{align} $$

  4. The lines are a system of equations, to be solved or otherwise treated together, e.g. $$ \begin{align} 2x &+ 4y &= 4 \\ x &+ y &= 3 \end{align} $$

(I would think that #3 is done because it looks tidy, whereas #1 and #2 are about continuity between lines, and #4 is more general related-ness. But that’s just my impression.)

I’m intentionally leaving out inequalities, by the way. They have their own issues.

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  • $\begingroup$ I would broadened your third category to systems of equations or inequalities. $\endgroup$
    – hardmath
    Commented Dec 11, 2022 at 4:21
  • $\begingroup$ I think it's a matter of preference. I've gotten away with writing something like $\text{something} = \text{other thing} = \text{this thing} = \text{that thing}$ in one begin-align environment on my assignments. If I find a line of equalities to have a lot of steps, I use the & signs to line up my equal signs. $\endgroup$
    – Accelerator
    Commented Dec 11, 2022 at 4:37
  • $\begingroup$ @hardmath I’m adding systems of equations as its own category. Thanks! $\endgroup$
    – Tim Pederick
    Commented Dec 11, 2022 at 5:02
  • $\begingroup$ @MarianoSuárez-Álvarez If ISO can tell us we should be using roman d, not italic, for differentials, it won’t surprise me in the least if they have something on when to align equals signs! $\endgroup$
    – Tim Pederick
    Commented Dec 11, 2022 at 5:20
  • $\begingroup$ I’d be interested to discuss the reasons for closing this as opinion-based. It’s not my intention to ask for anyone’s opinion on when they think equals signs should be aligned, nor even for anecdotal evidence of when they do see them aligned. I want to know of the factual existence of guidelines produced by any relevant organisation or body. I edited the question to remove anything I could find that I thought might be construed otherwise, but the response was simply that the issue was “not resolved”. $\endgroup$
    – Tim Pederick
    Commented Dec 18, 2022 at 4:54

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