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
, thoughaligned
's friendgathered
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 insidegather
oralign
environments), thesplit
andaligned
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 thataligned
(together withalignedat
andgathered
) 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 thetbtags
or (default)centertags
option to theamsmath
package. Whereasaligned
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 ofaligned
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 thecentertags
(default) ortbtags
option. Here is an exampleNow the same with uncommented
tbtags
:Now also
leqno
is uncommented: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 usesplit
whenever possible, if equation numbers are involved. (I rarely use equation numbers, so I usually don't bother.)