How to number math equations from both sides?

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:

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


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

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}


• (1) the math expression isn't centered on the page; it's centered between the equation number and the "tag". this has the result (2) if multiple instances of this structure are on the same page, they may have very different alignments. add a second, separate flalign with this string to see the difference: &&abc&=d + e + f&&\text{\footnotesize(a very long description)}. – barbara beeton Oct 14 '16 at 12:56
• you did better with the other answer (see comment above). but this is a better question, so if you fix it to use the answer shown there, i'll remove my comment and consider this for adding to the "math" section of "often referenced questions". – barbara beeton Oct 14 '16 at 13:07
• @barbara beeton: You're perfectly right. I've modified my answer. In my defence, I wrote this answer hastily, before I went to work… – Bernard Oct 14 '16 at 17:48
• hmmm. still a little refinement possible. question asks for ability to cross-reference, so a \label would be helpful. also, the other answer shows how to present a description that would overlap the main equation. and finally, if it were compilable (with \documentclass, etc.), that would put the icing on the cake. – barbara beeton Oct 14 '16 at 18:28
• @barbara beeton: Your wish is my command, milady! – Bernard Oct 14 '16 at 19:52