\documentclass{amsart}
\usepackage{amsmath,amssymb,nopageno}
\usepackage[all]{xy}
\begin{document}
 \begin{equation*}
  \xymatrix@R+2em@C+2em{
  A \ar[r]^-f \ar[d]_-g & B \ar[d]^-h \\
  C \ar[r]_-k & D
  }
 \end{equation*}
\end{document}

DescriptionCommutative square.svg	A generic commutative square
Date	14 January 2009
Source	I created it myself
Author	Ryan Reich
Permission (Reusing this file)	Public domainPublic domainfalsefalse I, the copyright holder of this work, release this work into the public domain. This applies worldwide. In some countries this may not be legally possible; if so: I grant anyone the right to use this work for any purpose, without any conditions, unless such conditions are required by law.
Other versions	w:File:Commutative square.svg