Good to use the arrays in the amsmath package
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Plain
\begin{matrix}
1 & 2 & 3\\
a & b & c
\end{matrix}
Parentheses/round brackets
\begin{pmatrix}
1 & 2 & 3\\
a & b & c
\end{pmatrix}
Brackets/square brackets
\begin{bmatrix}
1 & 2 & 3\\
a & b & c
\end{bmatrix}
Braces/curly brackets
\begin{Bmatrix}
1 & 2 & 3\\
a & b & c
\end{Bmatrix}
Pipes
\begin{vmatrix}
1 & 2 & 3\\
a & b & c
\end{vmatrix}
Double pipes
\begin{Vmatrix}
1 & 2 & 3\\
a & b & c
\end{Vmatrix}