Pokus

$\documentclass[border=3mm]{standalone}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
  \draw (0,0)node[below left]{A} -- 
  ++(2,0)node[below right]{C}coordinate(C) --
  ++(0,2)node[below right]{B} -- cycle;
  \draw (C) +(-3mm,0mm) |- +(0mm,3mm);
\end{tikzpicture}
\end{document}$