How can I position equation numbers always to the left side?

Question

Equations and amsmath environments put equation numbers to the right side by default, the output of

\begin{multline}
  some equation
\end{multline}

is:

equation (number)

How can I change it this way:

(number) equation

Answer 1

Pass the option leqno to your document class, to be used by the class and also inherited by packages, such as the amsmath package. It would already work if you would specify the option directly to amsmath, however for consistency I would already use it at class level.

\documentclass[leqno]{article}
\usepackage{amsmath}
\begin{document}
\begin{multline}
  some equation
\end{multline}
\end{document}

leqno means left equation numbers.