How to Put Two Multiline Brace Groups Around an Arrow in LaTeX

To place two multiline groups in braces with an arrow between them, put each group in an aligned environment and enclose it in a scalable left brace. Use \right. for an invisible matching delimiter; LaTeX needs a matching right delimiter whenever you write \left.

The example below shows one system being rewritten as another. It uses amsmath, which provides the aligned environment. You do not need eqnarray for this layout.

Complete example

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\[
\left\{
\begin{aligned}
x + y &= 5,\\
2x - y &= 1
\end{aligned}
\right.
\quad\longrightarrow\quad
\left\{
\begin{aligned}
x &= 2,\\
y &= 3
\end{aligned}
\right.
\]

\end{document}

The two rows in each aligned block break at \\. Put & before the part you want to align—in this example, it aligns the equals signs. Each \left\{ starts a brace that grows to fit the lines, and the corresponding \right. supplies an invisible delimiter on the right. The \quad commands add horizontal space around the arrow.

Why not use eqnarray?

The eqnarray environment is an older way to align equations, but its spacing around relation symbols such as = is often wider than standard mathematical spacing. The amsmath environments are more consistent and give you control over the alignment. For a multiline group inside a larger formula, aligned is a good fit because it can be used within display math alongside other symbols, including the arrow.

If you already have an eqnarray version, replace it with the aligned blocks shown above. Keep the whole expression inside one display-math environment so the braces, lines, and arrow are arranged as a single unit.

Variations

To show right braces instead of invisible right delimiters, use \right\} in place of \right.. To make the arrow shorter, use \to instead of \longrightarrow. If you want the arrow closer to the braces, reduce or remove the surrounding \quad spacing.

For a single multiline group with only a left brace, use the same pattern with one aligned block and \right.. If the lines are not equations to be aligned at a particular symbol, you can still use aligned; omit the alignment marker or place it where it gives the layout you want.

Common mistakes

• Leaving out \right.: \left and \right must be paired. An invisible delimiter is written as ..
• Using \left\{ without a multiline structure: The brace only sizes itself to the material inside it. Place the lines in aligned so they form one group.
• Putting the arrow on a new display line: Keep the arrow between the two groups inside the same display-math environment.
• Forgetting amsmath: Add \usepackage{amsmath} in the preamble; otherwise aligned may be undefined.
• Using eqnarray by default: It can work for some layouts, but aligned generally gives better spacing and clearer structure.