\documentclass{article}
\usepackage{booktabs}

\setlength{\parskip}{1em}

\begin{document}

A very basic booktabs table.

\begin{tabular}{lrc} \toprule
a & b & c \\ \midrule
d & e & f \\
g & h & i \\ \bottomrule
\end{tabular}

Let's change the width of the lines.

\begin{tabular}{lrc} \toprule[2pt]
a & b & c \\ \midrule[1pt]
d & e & f \\
j & k & l \\
g & h & i \\ \bottomrule[2pt]
\end{tabular}

Make the cmidrule only appear under cols one and two.

\begin{tabular}{lrc} \toprule[2pt]
a & b & c \\ \cmidrule[1pt]{1-2}
d & e & f \\
j & k & l \\
g & h & i \\ \bottomrule[2pt]
\end{tabular}

Trim the cmidrule left and right.

\begin{tabular}{lrc} \toprule[2pt]
a & b & c \\ \cmidrule[1pt](rl){1-3}
d & e & f \\
j & k & l \\
g & h & i \\ \bottomrule[2pt]
\end{tabular}

Add some extra space before the last row to indicate it is special.

\begin{tabular}{lrc} \toprule[2pt]
a & b & c \\ \cmidrule[1pt](rl){1-3}
d & e & f \\ 
j & k & l \\ \addlinespace[0.5em]
g & h & i \\ \bottomrule[2pt] 
\end{tabular}

\end{document}