# Environment aligned undefined when trying to write a system of equations

I'm trying to write a system of equations in LaTeX. From the Wiki page Advanced Mathematics I took this example:

\left.\begin{aligned} B'&=-\partial \times E,\\ E'&=\partial \times B - 4\pi j, \end{aligned} \right\} \qquad \text{Maxwell's equations}


but as a result I got:

! LaTeX Error: Environment aligned undefined.

See the LaTeX manual or LaTeX Companion for explanation.
Type  H <return>  for immediate help.
...

l.49  \left.\begin{aligned}


Does anybody know what goes wrong here?

• Please provide a minimal working example. We can definitely assist you better in that way. More than likely, from the error message, your missing the amsmath package which defines the aligned environment. – azetina Nov 14 '13 at 20:24

\usepackage{amsmath}