# Multiple labels for one line of alignment

I have these four equations thus aligned, and would like to label each of them separately, placing the labels to the left of the two equations on the left and to the right of the other two. So far, I did this:

\begin{align*}
\begin{split}
eq_1 (TL) \\
eq_2 (BL)
\end{split}
&&
\begin{split}
eq_3 (TR)
eq_4 (BR)
\end{split}
\end{align*}


My code is too full of personal commands for me to purge it, and besides the only relevant thing is the alignment and labeling, not the equations themselves. I tried putting \tag{…}\label{…} in the splits, but I got the error \tag not allowed here. I tried using gather* and aligned instead of split, getting the same error in the second case and the error \begin{gather*} only allowed in paragraph mode in the first. So how can I get those labels in place?

Update: Following David's answer, I tried the following:

\documentclass[a4paper]{report}
\usepackage{amsmath,mathptmx}
\makeatletter
\newcommand{\reqnomode}{\tagsleft@false\let\veqno\@@eqno}
\newcommand{\leqnomode}{\tagsleft@true\let\veqno\@@leqno}
\makeatother

\begin{document}
\begin{tabular}{@{}p{.4\textwidth}@{\hspace{.2\textwidth}}p{.4\textwidth}@{}}\leqnomode
$$\vec\nabla\cdot\vec E=\frac{\rho}{\epsilon_0} \tag{EDV}\label{eq:EDV}$$
&
\reqnomode
$$\vec\nabla\times\vec E+\partial_t\vec B=0 \tag{ECV}\label{eq:ECV}$$
\\[-20pt]
\leqnomode
$$\vec\nabla\cdot\vec B=0 \tag{MDV}\label{eq:MDV}$$
&
\reqnomode
$$\vec\nabla\times\vec B=\mu_0\vec J+\frac{1}{c^2}\partial_t\vec E \tag{MCV}\label{eq:MCV}$$
\end{tabular} \\
I really wanted to label these four equations as (EDV), (MDV), (ECV) and (MCV) for Electric Divergence in Vector form, Magnetic Curl in Vector form etc., but I seem to have trouble with \LaTeX. I will enquire on \TeX\ SX as to how I can achieve that. Let us now see the purported index form related to the tensor: \\
\begin{tabular}{@{}p{.4\textwidth}@{\hspace{.2\textwidth}}p{.4\textwidth}@{}}\leqnomode
$$\partial_iF^{0i}=\frac{1}{c\epsilon_0}\rho \tag{EDI}\label{eq:EDI}$$
&
\reqnomode
$$\epsilon_{ijk}\partial_jF_{k0}+\frac12\epsilon_{ijk}\partial_0F_{jk}=0 \tag{ECI}\label{eq:ECI}$$
\\[-20pt]
\leqnomode
$$\epsilon_{ijk}\partial_iF_{jk}=0 \tag{MDI}\label{eq:MDI}$$
&
\reqnomode
$$\partial_jF_{ij}=\mu_0J_i+\partial_0F_{i0} \tag{MCI}\label{eq:MCI}$$
\end{tabular}
\end{document}


Output:

Ignoring the sentences in the middle, and the indent problem which is easily fixed with \noindent, we have an evident horizontal alignment problem, as the TR equation in the vector forms is higher than its TL twin, and the BL index form strangely got aligned correctly, as opposed to my real situation where it was higher than the BR. So how do I fix the vector form? And any ideas why copy-pasting the code from another document where the index form was:

and only changing personal commands to (almost) equivalent standard commands I got the right alignment? I mean, my original code is:

\begin{tabular}{@{}p{.4\textwidth}@{\hspace{.2\textwidth}}p{.4\textwidth}@{}}\leqnomode
$$\pd_iF^{0i}=\xfr{1}{c\eg_0}\rg \tag{EDI}\label{eq:EDI}$$
&
\reqnomode
$$\eg_{ijk}\pd_jF_{k0}+\xfr12\eg_{ijk}\pd_0F_{jk}=0 \tag{ECI}\label{eq:ECI}$$
\\[-20pt]
\leqnomode
$$\eg_{ijk}\pd_iF_{jk}=0 \tag{MDI}\label{eq:MDI}$$
&
\reqnomode
$$\pd_jF_{ij}=\mg_0J_i+\pd_0F_{i0} \tag{MCI}\label{eq:MCI}$$
\end{tabular} \\


\pd is \partial, \eg,\mg,\rg are \epsilon,\mu,\rho plus a scale due to size mismatch between latin and greek in mathptmx, \xfr is \dfrac with raising of numerator and denominator, so why this difference?

• Maxwell equations in covariant form? ;-) – user31729 Mar 10 '15 at 8:28
• Indeed they are :). – MickG Mar 10 '15 at 8:55

\begin{tabular}{@{}p{.4\textwidth}@{\hspace{.2\textwidth}}p{.4\textwidth}@{}}
$$$$$$$$..$$$$$$$$
&
$$$$$$$$..$$$$$$$$
\\
$$$$$$$$..$$$$$$$$
&
$$$$$$$$..$$$$$$$$
\end{tabular}

• Sir, how about the positions of labels? – Symbol 1 Mar 10 '15 at 9:01
• Here's my code: i.stack.imgur.com/OAzNi.png. Output: i.stack.imgur.com/PrDjM.png – MickG Mar 10 '15 at 9:04
• Why is the bottom-left equation further down than its right-side twin? – MickG Mar 10 '15 at 9:05
• An analogous code for the vector form gives: i.stack.imgur.com/RJ86m.png. Whoops, should have shot only the equations. Anyway, the bottom is aligned, the top isn't, the right side being higher than the left. What's happening? – MickG Mar 10 '15 at 9:09

First I tried to set \newcommand{\LeftEqNo}{\let\veqno\@@leqno} but I failed due to vertical displacement in the first column. Therefore, I had to change to the align environment. Sadly, this environment adds more space above and below the equation. If you put the whole table in a group, you may define (\setlength{\abovedisplayskip}{0pt}...) whatever distance you like. It will not effect the rest of the document.

% arara: pdflatex
% arara: pdflatex

\documentclass{article}
\usepackage{mathtools}
\usepackage{tabularx}
\usepackage{array}

\makeatletter
\newcommand{\leqnomode}{\tagsleft@true}
\makeatother

\begin{document}
See equations \eqref{eqn:1}, \eqref{eqn:2}, \eqref{eqn:3}, and \eqref{eqn:4}!

\begingroup
\setlength{\abovedisplayskip}{0pt}
\setlength{\belowdisplayskip}{0pt}
\noindent
\begin{tabularx}{\textwidth}{@{}>{\leqnomode}XX@{}}
\begin{align}\label{eqn:1}\tag{TL}eq_1\end{align} & \begin{align}\label{eqn:2}\tag{TR}eq_3\end{align}\\
\begin{align}\label{eqn:3}\tag{BL}eq_2\end{align} & \begin{align}\label{eqn:4}\tag{BR}eq_4\end{align}
\end{tabularx}
\endgroup
\end{document}