%%
%% The LaTeX Companion, 3ed
%%
%% Example 6-9-9 on page I-504 in "Complex table headers".
%%
%% Copyright (C) 2022 Frank Mittelbach
%%
%% It may be distributed and/or modified under the conditions
%% of the LaTeX Project Public License, either version 1.3c
%% of this license or (at your option) any later version.
%%
%% See https://www.latex-project.org/lppl.txt for details.
%%

\documentclass{tlc3exa}
\pagestyle{empty}
\setcounter{page}{6}
\setlength\textwidth{183.0pt}
\usepackage{keyvaltable}
\NewKeyValTable[headformat=\bfseries, norowbg]{complexheader}
  { precolumn: align=c, format=\textbf ; colA: ; colB: ; colC: ;colD: }
  [ headers={ colA+colB+colC+colD : head=\textsf{From A to D} \\
              colA+colB : head= A \& B;
                       colC+colD : align=r, head=$\to$ C \& D \\ :: }
   ,colgroups={BtoE: span=colA+colB+colC+colD, format=\textit}        ]
%StartShownPreambleCommands

%StopShownPreambleCommands

\begin{document}
\begin{KeyValTable}{complexheader}
  \Row{ precolumn=1, colA=1.a, colB=1.b,
                           colC=---, colD=1.d }
  \Row{ precolumn=2, colA=2.a, colB=2.b,
                           colC=2.c, colD=2.d }
\Row[around=3pt]{BtoE=Centered subtitle}
  \Row{ precolumn=3, colA=3.a, colB=---,
                           colC=3.c, colD=3.d }
\end{KeyValTable}
\end{document}