Sometimes you want to write a sequence of steps, and write the explanation for each step next to it.
abc = xyz pythagoras rule
= uvw triangle inequality
= ABC
It is easy to do this with the amsmath package as detailed in this StackOverflow question.
\usepackage{amsmath}
\begin{align*}
abc &= xyz \\
&= uvw && \text{pythagoras rule} \\
&= D && \text{triangle inequality} \\
&= ABC &&
\end{align*}
