This is a wonderful way of making all sorts of commuutative diagrams in whatever area of mathametics you are working in. Here is a basic introduction to XY-pic. For more information and also for the updated version viz., xy' you should visit:

Guide to XY

1. Your latex document should contain the following line in the preamble: \usepackage{xypic}

2. Wherever you want to put the diagram, the body of the diagram should be within the following

$$\diagram ............. ............. \enddiagram$$
(Observe the unusual asymmetry in the two commands. It is not \begin{diagram}' that begins the diagram like so many latex environments.
Nor it is \end{diagram}'. A minor irratant, which is completely removed in the updated version xy'.)

3. Do not leave any blank lines in between.

4. The data in the body of the diagram is to be given in the form of a matrix, the columns of which are separated by
&' and the rows being separated by \\'.

5. All the main entries of a commutative diagram are automatically centered.

The connectors such as arrows and labels follow the main entry from which they emerge and should be typed before the next separator.

6. Let us begin with a fictitious example:

\documentstyle[12pt]{article}
\usepackage{xypic}
\begin{document}
$$\diagram & X\dlto\ddto &\\ V & & Y\dlto \\ & U & \enddiagram$$

\end{document}

Run the above document on your PC and see what you get.