# Double square bracket

I need to have 2 square bracket matrix on the same equation line. I'm able to do just for a single square bracket matrix (for \xi) with below

\xi = \left[ \begin{array}{c}
\xi^1  \\
\vdots \\
\xi^L
\end{array} \right]

Anyone can give me a hint how to add the second square bracket matrix (for x)?

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it is very recommended to use amsmath whenever one wants to typeset mathematics

\documentclass{minimal}
\usepackage{amsmath}
\begin{document}
$$\xi = \begin{bmatrix} \xi^1 \\ \vdots \\ \xi^L \end{bmatrix} \qquad \mathbf{x} = \begin{bmatrix} x^1 \\ \vdots \\ x^L \end{bmatrix}$$
\end{document}
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$$\xi = \left[ \begin{array}{c} \xi^1 \\ \vdots \\ \xi^L \end{array} \right]; \qquad \mathbf{x} = \left[ \begin{array}{c} x^1 \\ \vdots \\ x^L \end{array} \right]$$