\documentclass[12pt]{standalone}
\input{head}
\begin{document}
\begin{tikzpicture}\selectlanguage{english}
  \path (0,2) node(A){$a$} (2,2) node(SA){$Sa$} (4,2) node(TA){$Ta$};
  \path (0,0) node(B){$b$} (2,0) node(SB){$Sb$} (4,0) node(TB){$Tb$};
  \draw[->] (A) -- node[right]{$f$} (B);
  \draw[->] (SA) -- node[left]{$Sf$} (SB);
  \draw[->] (TA) -- node[right]{$Tf$} (TB);
  \draw[->] (SA) -- node[above]{$\tau_a$} (TA);
  \draw[->] (SB) -- node[below]{$\tau_b$} (TB);
\end{tikzpicture}
\end{document}
%%% Local Variables:
%%% mode: latex
%%% TeX-master: t
%%% End: