# Switch between leqno and reqno options (of amsmath) in the same document

I'd like to know if it is possible switch between the leqno and reqno options in the same document keeping the numbering of the equations. Namely i use the reqno option (default for amsmath and the standard article class), but i want that some equations are numbered by the left side (as if I had relied the leqno option) and the others no.

I'd like a commands as \leqnomode and \reqnomode to change the position of \tag in anywhere of the document.

Is this possible?

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Here you go:

\documentclass{article}
\usepackage[leqno]{amsmath}

\makeatletter
\newcommand{\leqnomode}{\tagsleft@true}
\newcommand{\reqnomode}{\tagsleft@false}
\makeatother
\begin{document}

\begin{align}
f(x) &= ax^2 + bx + c \\
g(x) &= dx^2 + ex + f
\end{align}

\reqnomode

\begin{align}
f(x) &= ax^2 + bx + c \\
g(x) &= dx^2 + ex + f
\end{align}

\end{document}

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What are these \makeatletter and \makeatother? – Enthusiastic Student Oct 16 '14 at 16:04
@EnthusiasticStudent: They are required when using @ symbols in macros. See What do \makeatletter and \makeatother do? – Werner Oct 16 '14 at 16:41
Is there a similar way of doing this with equation environment? – Mappi Dec 13 '14 at 18:00
@Mappi: I can't see a way to achieve that right now. A work-around is to use align, even though it has only a single equation and doesn't require any form of alignment. – Werner Dec 13 '14 at 19:22
@Werner: I tried to use your code in a (long) document, and it suppresses the equation number instead of changing on which side it is written. Do you have any idea which packages might be incompatible? – Bernard Feb 15 at 12:24