Multi-line equations with explanations on some lines

I would like something like this:

STH
= STH       REASONS Explained.
= STH       LOOOOOOOOOOOOOOOOOOONG
REASON
= STH.


How can I do this?

-

If you use the amsmath package then this can be done using the align (or align* if you don't want the equations numbered) environment together with the \text command for the text.

\documentclass{minimal}
\usepackage{amsmath}

\begin{document}
\begin{align*}
A &= B \\
&= C && \text{Because of D} \\
&= D && \text{But beware of E} \\
&&& \text{Some more on that}
&= F
\end{align*}
\end{document}


Result (note that the actual result will be centred on the page):

To get the alignment as you have it, you would have:

&A \\
&= B \\


at the start.

The amsmath package is loaded by default if you use one of the classes from the AMS, such as amsart.

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My reason bits are coming out right-aligned and I don't see how to left-align them :( – Reb.Cabin Mar 26 at 13:27
Ok, i figured this out: "reason" bits come out right-aligned if preceded by a single "&" and left-aligned if preceded by double "&&." – Reb.Cabin Mar 26 at 13:49