Commutative diagram

A simple example of a commutative diagram using TikZ, short and readable. It has been posted as answer to the question http://tex.stackexchange.com/q/45741/213 of Elias.

• A matrix is used for positioning the main nodes
• Arrows are drawn as edges, between the main nodes, using further nodes for labeling
• As columns and row distance is expressed by em units, which scale with the font size, the diagram can be scaled by using font size commands such as Large, huge etc.

Note the (m-2-1.east|-m-2-2) syntax for getting a horizontal arrow between cells of a different height.

Edit and compile if you like:

% A simple commutative diagram
% Stefan Kottwitz
\documentclass{article}
\usepackage{tikz}
\usepackage[active,tightpage]{preview}
\PreviewEnvironment{tikzpicture}
\setlength\PreviewBorder{5pt}%
\usetikzlibrary{matrix}
\begin{document}
\begin{tikzpicture}
  \matrix (m) [matrix of math nodes,row sep=3em,column sep=4em,minimum width=2em]
  {
     F_t(x) & F(x) \\
     A_t & A \\};
  \path[-stealth]
    (m-1-1) edge node [left] {$\mathcal{B}_X$} (m-2-1)
            edge [double] node [below] {$\mathcal{B}_t$} (m-1-2)
    (m-2-1.east|-m-2-2) edge node [below] {$\mathcal{B}_T$}
            node [above] {$\exists$} (m-2-2)
    (m-1-2) edge node [right] {$\mathcal{B}_T$} (m-2-2)
            edge [dashed,-] (m-2-1);
\end{tikzpicture}
\end{document}