% This picture represents the complete graph of four vertices
% Copyright Pablo (C) 2021
\begin{tikzpicture}[scale=0.75]
  % The vertices
  \coordinate (a) at (0,0);
  \coordinate (b) at (4,0);
  \coordinate (c) at (2,1);
  \coordinate (d) at (2,3);
  \foreach \node in {a,b,c,d}
  \filldraw (\node) circle (2pt);

  % The edges
  \draw (a) -- (b) 
            -- (c) 
            -- (a) 
            -- (d) 
            -- (b) 
            -- (c) 
            -- (d);
\end{tikzpicture}
