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\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}