\hline ¶Draw a horizontal line the width of the enclosing tabular or
array environment. It’s most commonly used to draw a line at the
top, bottom, and between the rows of a table.
In this example the top of the table has two horizontal rules, one above
the other, that span both columns. The bottom of the table has a single
rule spanning both columns. Because of the \hline, the
tabular second row’s line ending double backslash \\
is required.
\begin{tabular}{ll} \hline\hline
Baseball &Red Sox \\
Basketball &Celtics \\ \hline
\end{tabular}