# Equation tags for parts of alignments

Consider the following example:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
1+2=3 \implies \left\{ \begin{aligned} &a+b=c, \\ &x=y+z. \end{aligned} \right.
\end{document}


Is there a way to set up two equation tags, one per line in the aligned environment?

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You can do this by using the empheq package from the mh bundle. See code below for an example.

\documentclass[11pt,a4paper]{article}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{empheq}

\begin{document}
\begin{empheq}[left={1+2=3\implies\empheqlbrace}]{align}
a+b &= c \\
x &= y+z
\end{empheq}
\end{document}


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Just a different way of obtaining the same result:

\documentclass{article}
\usepackage{amsmath}% http://ctan.org/pkg/amsmath
\begin{document}
\begin{align}
\smash{
\raisebox{\dimexpr-.5\normalbaselineskip-.5\jot}
{$1+2=3\implies\left\{\begin{array}{@{}c@{}}\null\\\null\end{array}\right.$}}
a+b &= c, \\
x &= y+z.
\end{align}
\end{document}


The key here is to set the "left-hand side" separately from the "right-hand side" and align the two using some vertical movement.

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Note that this works great only if the two lines have "usual" height; e.g., it breaks if they contain fractions. –  Hendrik Vogt Oct 13 '12 at 6:47