[Tex/LaTex] What’s the difference between split and aligned


I don't understand the difference between the split and aligned environments introduced by amsmath, so I'm posting this question as a place to collect what subtle details there may be. One thing observable straight off is that split only works with two columns, whereas aligned works with an arbitrary number of columns. But they are similar in other immediately visible ways:

  • Both have to be wrapped in another math-introducing environment, like \begin{equation}...\end{equation}
  • Both permit only a single \tag{...} for the entire group, not one tag for each line (EDIT: not sure where I got that impression, in fact neither permits any \tag, though aligned's friend gathered does permit a single \tag, as described here)

I searched existing questions on this site about what differences there may be between these environments. I did find this question: Difference between (split, align) and (gather, aligned)?, where—despite the title and phrasing of the question—most of the answers focus just on the difference between the split and aligned environments themselves.

EDIT: Collecting the differences noted so far.

  • As one answer to the above question points out, inside equation environments (but not inside gather or align environments), the split and aligned environments have different vertical spacing from the surrounding text.

  • As another answer to that question points out, when the body of these environments gets very long, their horizontal placement starts to diverge.

  • As Mico noted in comments, section 3.7 of the amsmath User Guide notes that aligned (together with alignedat and gathered) accepts an optional [t] or [b] argument, for explicit vertical placement of any equation tags. split does not accept any such argument.

  • As egreg notes in his answer, split will on the other hand honor the tbtags or (default) centertags option to the amsmath package. Whereas aligned and its friends will not.

  • As Mico's comments also suggested, but I did not immediately appreciate, split is not supposed to go together with any other typeset material on the same display line. On the other hand, aligned and its friends can be freely combined with other unaligned materials, or even other blocks of aligned and so on. They will be horizontally juxtaposed and vertically centered. I've explained this further in an answer below.

Perhaps that exhausts the differences between split and aligned: though if others know of other differences, please point them out.

Best Answer

One important difference is that split obeys to the centertags (default) or tbtags option. Here is an example

%  tbtags,
%  leqno,

enter image description here

Now the same with uncommented tbtags:

enter image description here

Now also leqno is uncommented:

enter image description here

To the contrary, aligned will have the equation number according to the vertical alignment option: centered for \begin{aligned}...\end{aligned}, at the top for \begin{aligned}[t]...\end{aligned}, at the bottom for \begin{aligned}[b]...\end{aligned}. Thus it's better to use split whenever possible, if equation numbers are involved. (I rarely use equation numbers, so I usually don't bother.)

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