If there are fractions in the matrix,
. . they may be too close together vertically.
[tex]\begin{pmatrix}
\frac{1}{2} & \frac{1}{3}\\
\frac{1}{4} & \frac{1}{5}
\end{pmatrix}[/tex]
. .
We can insert an extra vertical space with \\[-4mm]
[tex]\begin{pmatrix}
\frac{1}{2} & \frac{1}{3} \\ \\[-4mm]
\frac{1}{4} & \frac{1}{5}
\end{pmatrix}[/tex]
. .
Using \\[-3mm] results in a larger space
. .