\begin{bmatrix}
   \dfrac{\partial x_1}{\partial y_1} & \dfrac{\partial x_2}{\partial y_1}
\\ ~
\\ \dfrac{\partial x_1}{\partial y_2} & \dfrac{\partial x_2}{\partial y_2}
\end{bmatrix}

\begin{pmatrix}
   1 & 2 \\
   3 & 4
\end{pmatrix}

\begin{Bmatrix}
   1 & 2 \\
   3 & 4
\end{Bmatrix}

\begin{vmatrix}
   1 & 2 \\
   3 & 4
\end{vmatrix}

\begin{Vmatrix}
   1 & 2 \\
   3 & 4
\end{Vmatrix}

