Table with weird borders with the booktabs package

Question

I'd like a table that looks like this in Beamer:

But the best I can do is this:

\documentclass{beamer}
\usepackage{booktabs}
\begin{document}
\frame{
\begin{tabular}{|c|c||c|}\cmidrule{2-3}\noalign{\vskip-\belowrulesep}
\multicolumn{1}{c|}{} & $M_2$ & $a$\\\hline
$M_1$ & $M_3$ &\\\noalign{\vskip-\aboverulesep}\cmidrule{1-2}
    \morecmidrules\cmidrule{1-2}\noalign{\vskip-\belowrulesep}
$b$ & \multicolumn{1}{c}{} & \multicolumn{1}{c|}{$x$}\\\hline
\end{tabular}
}
\end{document}

Is the table I want possible?

Answer 1

A solution with the tabularray package:

\documentclass{beamer}
\usepackage{tabularray}
\begin{document}
\frame{
    \begin{tblr}{
        colspec = { *{3}{c }},
        hline{1} = {1}{2-3}{},
        hline{2,4} = {1}{1-3}{},
        hline{3} = {1}{1-2}{rightpos = 0},
        hline{3} = {2}{1-2}{},
        vline{1} = {1}{2-3}{},
        vline{2} = {1}{1-3}{},
        vline{3} = {1}{1-2}{},
        vline{3} = {2}{1-2}{belowpos = 1},
        vline{4} = {1}{1-3}{belowpos = 1},
    }
     & $M_2$ & $a$ \\ 
    $M_1$ & $M_3$ & \\
    $b$ &  & $x$ \\
    \end{tblr}
}
\end{document}

---

If you want to get a continuous gap also between the vertical lines on the right side, it might be better to use the nicematrix package, which combines the strengths of TikZ with the possibilities of tabulars (at least tabularray won't give you a nice result in this case):

\documentclass{beamer}
\usepackage{nicematrix, tikz}
\begin{document}
\frame{
    \begin{NiceTabular}{ *{3}{c} }
     & $M_2$ & $a$ \\ 
    $M_1$ & $M_3$ & \\ 
    $b$ &  & $x$ \\
    \CodeAfter
        \begin{tikzpicture}
            \draw 
                (1-|2) -- 
                ([xshift=-1pt]1-|3) -- 
                ([xshift=-1pt, yshift=1pt]3-|3) -- 
                ([yshift=1pt]3-|1) -- 
                (2-|1) -- (2-|2) -- cycle;
            \draw 
                ([xshift=1pt]1-|3) -- 
                (1-|4) -- (4-|4) -- (4-|1) --
                ([yshift=-1pt]3-|1) -- 
                ([xshift=1pt, yshift=-1pt]3-|3) -- 
                cycle;
            \draw (2-|2) -- ([xshift=-1pt]2-|3)
                ([xshift=1pt]2-|3) -- (2-|4);
            \draw (2-|2) -- ([yshift=1pt]3-|2) 
                ([yshift=-1pt]3-|2) -- (4-|2);
        \end{tikzpicture}
    \end{NiceTabular}
}
\end{document}

Answer 2

Tikz for you :-)

\documentclass{beamer}
\usepackage{tikz}
\usetikzlibrary{positioning}
\begin{document}
\begin{frame}
  \begin{tikzpicture}[
    , thick
    , n/.style = {
      , minimum width = 10mm
      , minimum height = 6mm
      , draw
    }
    , outer sep = 0
    , node distance = 0 and 0
  ]
    \node (M2) [n] {$M_2$};
    \node (M3) [below = of M2, n] {$M_3$};
    \node (M1) [left = of M3, n] {$M_1$};
    \node (a) [right = 1mm of M2, n] {$a$}; % add , minimum width = 7mm if you want narrower node
    \node (b) [below = 1mm of M1, n] {$b$};
    \node (x) [below = 7mm of a.south, minimum height = 6mm] {$x$};
    \draw (M2.north east) -- (a.north west);
    \draw (M2.south east) -- (a.south west);
    \draw (a.south west) |- (b.north east);
    \draw (a.south east) |- (b.south east);
  \end{tikzpicture}%
\end{frame}
\end{document}

Answer 3

With hhline package;

\documentclass{beamer}
\usepackage{hhline}

\begin{document}
\begin{frame}
\begin{center}
\renewcommand\arraystretch{1.2}
    \begin{tabular}{|c|c||c|}
    \hhline{~|-|-|}
\multicolumn{1}{c|}{}
        & $M_2$ & $a$   \\
    \hhline{-|--|}
$M_1$   & $M_3$ &       \\
    \hhline{:=:=:b|~|}
$b$     & \multicolumn{1}{c}{}
                &  $x$  \\
    \hline
    \end{tabular}
\end{center}
\end{frame}
\end{document}

Answer 4

A little patch to a simple tabular with the help of array to play with the intercolumn spaces (@{}):

\documentclass{article}
\usepackage{array}
\begin{document}\sffamily\extrarowheight2pt
\begin{tabular}{|c|c|@{}p{2pt}@{}|l|}
\cline{2-4}
\multicolumn{1}{c|}{}&M2&&a\\\hline
M1&M3&&\\\cline{1-2}
\multicolumn{3}{c}{\vspace*{-12pt}}&%
\multicolumn{1}{@{\kern-.4pt\rule[8pt]{.4pt}{2.6pt}}c|}{}\\\cline{1-3}
b & \multicolumn{2}{c}{} &  x \\
\hline
\end{tabular}
\end{document}