# Labeling equations or the use of subequations

I am using the tabbing environment for a list of 5 related equations.

\begin{tabbing} \hspace{.5cm} \= \hspace{.3cm} \= \hspace{3cm} \= \kill
$$\mathbf{\Omega}$$ \> $$=$$ \> $$\Omega\hat{\mathbf{k}}$$
\tagthisline\\[.3cm]
$$\mathbf{r}$$ \> $$=$$ \>
$$x\hat{\mathbf{i}} + y\hat{\mathbf{j}} + z\hat{\mathbf{k}}$$\tagthisline
\\[.3cm]
$$\dot{\mathbf{r}}$$ \> $$=$$ \>
$$\mathbf{v}_{\text{CG}} + \mathbf{\Omega}\times\mathbf{r} + \mathbf{v}_{\text{rel}}$$\tagthisline\\[.3cm]
$$\mathbf{v}_{\text{rel}}$$ \> $$=$$ \>
$$\dot{x}\hat{\mathbf{i}} + \dot{y}\hat{\mathbf{j}} + \dot{z}\hat{\mathbf{k}}$$\tagthisline\\[.3cm]
$$\mathbf{a}_{\text{rel}}$$ \> $$=$$ \>
$$\ddot{x}\hat{\mathbf{i}} + \ddot{y}\hat{\mathbf{j}} + \ddot{z}\hat{\mathbf{k}}$$\tagthisline
\end{tabbing}


\tagthisline picks up the appropriate numbering from the document. However, I need a \label for each one in the tabbing environment.

1. Does assigning more than one label in the same environment work?

2. Even if it does, should a different method be taken here?

3. If the answer to (2) is yes, how should I set it up?

Without knowing what \tagthisline looks like, I would opt for a different approach using a straight align:

\documentclass{article}
\usepackage{mathtools}% http://ctan.org/pkg/mathtools
\providecommand{\tagthisline}{X}% Don't know what this means...
\begin{document}
\begin{tabbing} \hspace{.5cm} \= \hspace{.3cm} \= \hspace{3cm} \= \kill
$$\mathbf{\Omega}$$ \> $$=$$ \> $$\Omega\hat{\mathbf{k}}$$
\tagthisline\\[.3cm]
$$\mathbf{r}$$ \> $$=$$ \>
$$x\hat{\mathbf{i}} + y\hat{\mathbf{j}} + z\hat{\mathbf{k}}$$\tagthisline
\\[.3cm]
$$\dot{\mathbf{r}}$$ \> $$=$$ \>
$$\mathbf{v}_{\text{CG}} + \mathbf{\Omega}\times\mathbf{r} + \mathbf{v}_{\text{rel}}$$\tagthisline\\[.3cm]
$$\mathbf{v}_{\text{rel}}$$ \> $$=$$ \>
$$\dot{x}\hat{\mathbf{i}} + \dot{y}\hat{\mathbf{j}} + \dot{z}\hat{\mathbf{k}}$$\tagthisline\\[.3cm]
$$\mathbf{a}_{\text{rel}}$$ \> $$=$$ \>
$$\ddot{x}\hat{\mathbf{i}} + \ddot{y}\hat{\mathbf{j}} + \ddot{z}\hat{\mathbf{k}}$$\tagthisline
\end{tabbing}

\newcommand{\Lalign}{\phantom{\mathbf{a}_{\text{rel}}}}
\begin{align}
\mathrlap{\mathbf{\Omega}}\Lalign &= \Omega\hat{\mathbf{k}} \\[\jot]
\mathrlap{\mathbf{r}}\Lalign &= x\hat{\mathbf{i}} + y\hat{\mathbf{j}} + z\hat{\mathbf{k}} \\[\jot]
\mathrlap{\dot{\mathbf{r}}}\Lalign & =
\mathbf{v}_{\text{CG}} + \mathbf{\Omega}\times\mathbf{r} + \mathbf{v}_{\text{rel}} \\[\jot]
\mathbf{v}_{\text{rel}} &= \dot{x}\hat{\mathbf{i}} + \dot{y}\hat{\mathbf{j}} +
\dot{z}\hat{\mathbf{k}} \\[\jot]
\mathbf{a}_{\text{rel}} &= \ddot{x}\hat{\mathbf{i}} + \ddot{y}\hat{\mathbf{j}} +
\ddot{z}\hat{\mathbf{k}}
\end{align}
\end{document}


Now you can \label and \eqref to your heart's content.

If you wish to have a flush-left alignment, use flalign together with an additional alignment character & at the end of one of the equations. Also, for obtaining a similar lcl alignment to that of your tabbing environment, you can adjust the regular align preamble (\align@preamble). I've done the necessary adjustment in the definition of a new environment lflalign below:

\documentclass{article}
\usepackage{amsmath}% http://ctan.org/pkg/amsmath
\makeatletter
\newenvironment{lflalign}{%
\def\align@preamble{%
&\strut@
\setboxz@h{\@lign$\m@th\displaystyle{####}$}%
\ifmeasuring@\savefieldlength@\fi
\set@field
\hfil
\tabskip\z@skip
&\setboxz@h{\@lign$\m@th\displaystyle{{}####}$}%
\ifmeasuring@\savefieldlength@\fi
\set@field
\hfil
\tabskip\alignsep@
}
\flalign}
{\endflalign}
\makeatother
\begin{document}
\begin{flalign}
f(x) &= ax^2 + bx + c &\\
f(x)+g(x) &= d
\end{flalign}

\begin{lflalign}
f(x) &= ax^2 + bx + c &\\
f(x)+g(x) &= d
\end{lflalign}
\end{document}

• So align can have each line labeled? Then can IEEEeqnarray have each line labeled too? Commented Jun 14, 2013 at 1:05
• @dustin: Yup...
– Werner
Commented Jun 14, 2013 at 1:15
• That is what I will probably use than. You don't need to change your answer though. Commented Jun 14, 2013 at 1:16
• How can I left align IEEEeqnarray? Is flalign an added option in []? Commented Jun 14, 2013 at 2:35
• that is great. What do you read to obtain this information on how to change the environments? Commented Jun 14, 2013 at 14:51