Preserve spacing while using align with amsmath

Question

While

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align}
&   A \\
& = B
\end{align}
\end{document}

preserves the horizontal space after the equals sign,

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align}
  & A \\
= & B
\end{align}
\end{document}

gobbles this. Is there a way to preserve the space? (OK, in the MWE alignment could be placed behind A and B, but it is just a minimal example.) If it is not: How wide is the space (as to use \hspace{ something })?

Answer 1

in order to be able to add the correct space, (la)tex needs to know exactly what precedes and follows the equals sign. in this case, the & is "hiding" the letter that follows, so the space added is the default -- none.

in most cases, signs of relation are preceded and followed by "ordinary" characters (like letters). you can emulate this situation in your example by adding an empty group -- {} before and/or after the equals sign:

{} = {} & B

this is actually overkill; only the group after the equals sign is needed, but the extra group doesn't hurt, and may make the intention more clear to a reader of the input.

if you are aligning on a character of some other class, the "correct" spacing is shown in a little table in the texbook, p.170.

Answer 2

The AMS alignments are designed so that you use the & before not after the alignment operator, If you really can't do that for some reason using {}={} rather than = will ensure the equals sign gets the infix relation spacing.