# Problem with aligned environment in latex?

`aligned`

takes an optional argument and so it is mistakening `[X]`

for that option. The way to overcome is to place something innocuous after the `{aligned}`

, but before the `[X]`

. Here I place an empty group `{}`

.

Note that the option, when employed, is a vertical-alignment positioning specification, such as `[t]`

or `[b]`

.

```
\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{equation}
\begin{aligned}{}
[X] &= 1\\
[Y] &= 2,
\end{aligned}
\end{equation}
\end{document}
```

Another solution is to load the mathtools package. The `mathtools`

package is a (very significant!) superset of the `amsmath`

package which also fixes a couple of *de facto* bugs -- such as mis-interpreting `[X]`

as the optional argument of `\begin{aligned}`

-- of the `amsmath`

package. For more about this bug, see section 2.1 of the user guide of the `mathtools`

package.

To wit,

```
\documentclass[leqno]{article}
\usepackage{mathtools}
\begin{document}
\begin{equation}
\begin{aligned}
[X] &= 1\\
[Y] &= 2,
\end{aligned}
\end{equation}
\end{document}
```

works exactly as expected:

A nice aspect of this solution is that you needn't remember to insert `{}`

after `\begin{aligned}`

if the next line begins with an object -- such as `[X]`

-- that's encased in square brackets.