How to number math equations from both sides?

Question

What I want to achieve looks something like this:

(1) E = mc^2 (equivalence mass-energy)

i.e., I want to have a tag on the right-side and general number on the left-side.

The initial approach serves only half of my needs:

\begin{equation}\label{eq:equivalence-mass-energy}
E=mc^2 \tag{equivalence mass-energy}
\end{equation}

And then, how can I control what will be shown (number or tag) in cross-reference?

Answer 1

One of these his should do the trick:

\documentclass{article}
\usepackage[utf8]{inputenc}%
\usepackage[leqno]{amsmath}
\usepackage{nccmath} %
\usepackage{cleveref}

\begin{document}

\begin{flalign}
  &&E&=mc²&&\llap{\footnotesize(equivalence mass-energy)}\label{Ein}\\
  & & L & = m\Bigl(c²-\mfrac12 v²\Bigl) & & \llap{\footnotesize\parbox[t]{0.25\linewidth}{(Poincaré's equation for the lagrangian of an electron)}}\label{Zwei}%
\end{flalign}
\Cref{Ein} is essentially the same as \cref{Zwei}. %

\begin{alignat}{2}
  \hspace{6.5em} E & =mc² & \hspace{6.5em} & \rlap{\footnotesize(equivalence mass-energy)}\label{Ein-a} \\
  L & = m\Bigl(c²-\mfrac12 v²\Bigl) & & \rlap{\footnotesize\parbox[t]{0.25\linewidth}{(Poincaré's equation for the lagrangian of an electron)}}\label{Zwei-a}%
\end{alignat}
Another layout for \cref{Ein-a,Zwei-a}. %

\end{document}