# Align (environment)

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The align environment is used for two or more equations when vertical alignment is desired; usually binary relations such as equal signs are aligned.

American Mathematical Society, User's Guide for the amsmath Package

For all intents and purposes, it's a replacement for the eqnarray environment and all its warts. Rather than

\begin{eqnarray*}
x^2 + y^2 &=& 1
\\ y &=& \sqrt{1 - x^2},
\end{eqnarray*}


## one can type Edit

\begin{align*}
x^2 + y^2 &= 1 \\
y &= \sqrt{1 - x^2}.
\end{align*}


## Benefits over eqnarray Edit

Besides the slightly simpler syntax, you side-step bugs documented by Lars Madsen for The PracTeX Journal:

• Inconsistent spacing around binary symbols like 

 = and +;

• Overwriting equation numbers; and
• Silent label mismatch.

## Multiple equations on one line Edit

Besides being used for aligning binary symbols, the ampersand can also mark an invisible alignment for separating columns of equations. For example,

\begin{align}
u  &= \arctan x             &  dv &= 1 \, dx  \\
du &= \frac{1}{1 + x^2} dx  &  v  &= x.
\end{align}


produces:

\begin{align} u &= \arctan x & dv &= 1 \, dx \\ du &= \frac{1}{1 + x^2} dx & v &= x. \end{align}

## Preamble Edit

To use align, import the amsmath package in your preamble.

.....
\usepackage{amsmath}
.....

\begin{document}
\begin{align*}
u  &= \arctan x             &  dv &= 1 \, dx  \\
du &= \frac{1}{1 + x^2} dx  &  v  &= x.
\end{align*}
\end{document}