align vs. equation: Which way is typograpically preferred?

Question

The main differences between align and equation are the spaces and that align cannot be used with split, as far as I know.

What would be, from a typographic point of view, the preferred practice? What reasons are there for:

• mixing equation and align in one document (i.e. using equation for one line equations and align for multiline ones),
• using only equation (with split for multilines),
• using only align,
• using other possibilities?

Edit

If it's intended that one uses more space when you have multiline equations i.e. if it's preferred to mix align and equation, then one might ask further: Why isn't for example align implemented in this way, i.e. why is it that it detects if it's a single line equation or a multiline one and add the appropriate space before and after the equation?

Answer 1

For the typographic comparison you're looking to make, we need to distinguish between two groups of environments that generate display-style equations:

• Methods for generating (mostly) single-line equations, such as the \[ ... \] approach and the displaymath, equation, and equation* environments.

• Methods for generating multi-line equations (although they can be abused to create single-line equations), such as the gather, align, and multline environments.

The reason this distinction is important from a typographic point of view is that the latter group always inserts \abovedisplayskip and \belowdisplayskip above and below the respective environment. In contrast, the former group checks the length of the line that comes immediately before the environment: if the line is short, \abovedisplayshortskip and \belowdisplayshortskip are inserted instead of \abovedisplayskip and \belowdisplayskip. Only if the preceding line is long do the latter two length parameters come into play.

The consequence is that with \[ ... \] approach and the displaymath, equation, and equation* environments, LaTeX is given a chance to typeset the math material more compactly -- something that cannot happen if gather and/or align environments are (mis-)used for single-line displayed equations.

Another important aspect is that no page breaks are possible inside split and aligned environments, whereas it's possible to have page breaks inside gather and align environments (e.g., by issuing the instruction \allowdisplaybreaks).

Answer 2

Reading the amsmath package documentation you'll find that split (only usable within another environment) is for splitting a single equation on more than one line (e.g. when the equation is too long), while the align environment is for typesetting multiple equation (possibly related to one another) aligned in the same display environment

I think this answers your question: depending on what you have to typeset, you can choose the best environment.

Answer 3

Do you want an equation? Then use equation or \[ .. \] (or equation* if you want it not to be numbered). Do you want more than one equation together with no text in between? Then use gather(*) (if no alignment is necessary) or align(*) if you want to align them at certain points. To me that's the way of deciding what to use.

Now, if it's a single equation that has to be split over a few lines, then use split inside it. If you need a particular alignment inside that equation (for instance defining a map \!\begin{aligned} f \colon A &\to B \\ x &\mapsto f(x) \end{aligned} or use multlined if a part of the equation is too long and has to be split).

Of course, may be there are exceptions, but I think this is a straightforward way of choosing (rather than choosing, understanding) what environments to use.