# Tag placement in split equation within alignment block

Whenever I have an equation that does not fit on a single line, I am using $$\begin{split}...\end{split}$$ as recommended in the AMS math guide. This gives me an equation number centered vertically over the whole block. Fine.

1. However, what is the best practice when such a split equation is part of a larger alignment block?

Is it still OK to use

• Variant A

\begin{align}
\text{First} & = \text{line} \\
\begin{split}
& = \text{Second line \textellipsis} \\
& \phantom{{}={}} \text{\textellipsis continued}
\end{split}
\end{align},


• or better: Variant B

\begin{align}
\text{First} & = \text{line} \\
& = \text{Second line \textellipsis} \notag \\
& \phantom{{}={}} \text{\textellipsis continued}
\end{align},


• or even: Variant C

\begin{align}
\text{First} & = \text{line} \\
& = \text{Second line \textellipsis}  \\
& \phantom{{}={}} \text{\textellipsis continued} \notag
\end{align}?


2. What if the split equation contains three lines, and the vertically centered equation number pretty much aligns with the second line of the split equation in Variant A:

3. If you prefer Variants B or C, should the split environment be avoided (via multline, say) in the single equation case, too, for the sake of consistency?

I know this is more of a typography question, but since it is math  typography, I feel it is better asked here.

-
I personally use variant C above, with \nonumber\\ instead of just \\ at the end of each incomplete line. –  Nicholas Hamilton Dec 15 '12 at 16:28
\usepackage[tbtags]{amsmath} will put the tag of a split equation at the bottom (if equation numbers are to the right) or the top (if equation numbers are to the left). –  Aditya Dec 17 '12 at 5:01

I believe that assigning two equation numbers to one long equation First = line = Second line continued is a non-sense, and even if it makes sense, it will always confuse the reader. In my opinion, the correct solution is to First or line and put an explanatory line in between:

\begin{align}
First ={}& line
\intertext{and by the Piegonhole principle}
\begin{split}
First ={}& Secondline % choose one of these two lines
%   line  ={}& Secondline % choose one of these two lines
\\
& continued
,\end{split}
\end{align}


Notice that you can use ={}& to place the alignment position after = and get the proper spacing.

(I decided to make the texts math-mode to 1st show how ugly it is and 2nd show that the spacing around the ='s is correct.)

As for the vertical alignment of the number, there's no real preference of one or the other. Just notice that if you put it on the bottom line and it's on the right, the correct left variant is on the top line.

\begin{align}
First ={}& line
\intertext{and by the Piegonhole principle}
\nonumber
First ={}& Secondline % choose one of these two lines
% line  ={}& Secondline % choose one of these two lines
\\
& continued
,\end{align}

-
You might be right regarding a continued long equation, but my question also applies to the case First = line \\ second = line \\ continued, if you know what I mean. Regarding alignment after =, I still prefer my phantom solution, as it is local, i.e., does not affect the other lines, that otherwise have this ugly empty group. –  mafp Dec 12 '12 at 23:14
Sorry, I was misleading in the previous version of the commen. The vertical placement is up to you, there are two options, either center it (like I did in the example) or place it at the bottom-most line. The only other rule is that you should be consistent throughout the whole document. And of course, if your number is on the left, you put it on the top instad of bottom, however, centered is still centered. See the updated answer for details. –  yo' Dec 13 '12 at 13:59