Sorry, if this question sounds a bit amateurish, but I haven't been able to find a good summary about the differences between these four environments for multi-lining an equation. Where do they differ and which one should I choose under different circumstances?

# [Tex/LaTex] the difference between split, multline, align, breqn for breaking an equation into multiple lines

alignbreqnequationsmath-modemultline

#### Related Solutions

First of all, the problem presents for *textual* subscripts, such as those used in physics to distinguish between vectors with the same name (say a force) by a subscripted label that should go in upright type. Textual subscripts are used in many other fields.

In what follows, `amsmath`

is assumed.

`$W_{\rm total}$`

is totally wrong as it relies on a deprecated command that classes don't need to define (and indeed some don't).`$W_{\mathrm{total}}$`

is the correct form of the above. Limitations: spaces are gobbled and hyphens become minus signs.`$W_{\textnormal{total}}$`

uses the main roman font of the document, no matter the context; the argument is typeset as text at the correct size.`$W_{\mathup{total}}$`

(with Ulrike Fischer's definition) has one advantage over`\mathrm`

, since it uses`\familydefault`

, but the same limitations.`$W_{\operatorname{total}}$`

is like using a sledgehammer for killing a fly. It's the same as`\mathrm`

, but hyphens don't become minus signs.`$W_{\text{total}}$`

might seem ideal, but it changes font according to the context, so the subscript would appear in italics in a theorem statement.

Therefore, form 3 seems the most natural. Notice that braces are not really necessary, except in case 5.

To be honest, for single words `\mathrm`

(or `\mathup`

) is more efficient, as `\textnormal`

uses `\mathchoice`

and typesets four times the subscript in different sizes. However, the overhead is almost negligible with modern machines and uniformity is to be preferred to efficiency, when it doesn't slow the workflow in a significant way.

If, for some reasons, one wants that textual subscripts are typeset in upright type, but keeping the current font family, for instance because some parts of the document use sans serif type also for math (which I don't agree with), a modified version of `\textnormal`

can be used:

```
\makeatletter
\DeclareRobustCommand{\textnormalf}[1]{% f for "keep the family
\text{\usefont{\f@encoding}{\f@family}{m}{n}#1}%
}
\makeatother
```

Here `\f@encoding`

and `\f@family`

are the current output font encoding and font family, as stored by LaTeX at each (text) font change; with font series `m`

and font shape `n`

we're choosing upright medium type.

Of course, a more meaningful name for `\textnormalf`

should be chosen according to its usage and semantics.

One important difference is that `split`

obeys to the `centertags`

(default) or `tbtags`

option. Here is an example

```
\documentclass[twocolumn]{article}
\usepackage[
% tbtags,
% leqno,
]{amsmath}
\begin{document}
\begin{align}
a&=b\\
\begin{split}
c&=d+{}\\
&=e+{}\\
&=f
\end{split}\\
g&=h
\end{align}
\end{document}
```

Now 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 use `split`

whenever possible, if equation numbers are involved. (I rarely use equation numbers, so I usually don't bother.)

## Best Answer

As noted in comments the

`amsmath`

and`breqn`

documentation have several good examples, also the`mathtools`

package has extended versions of several of the`amsmath`

alignments. But the usual style here is to answer inline rather than refer to manuals, so this is a document giving the basic usage of the environments you mention.