# LaTeX/Xy-pic

xy is a special package for drawing diagrams. To use it, simply add the following line to the preamble of your document:

```\usepackage[all]{xy}
```

where "all" means you want to load a large standard set of functions from Xy-pic, suitable for developing the kind of diagrams discussed here.

The primary way to draw Xy-pic diagrams is over a matrix-oriented canvas, where each diagram element is placed in a matrix slot:

 ```\xymatrix{A & B \\ C & D } ```

The \xymatrix command puts its contents in math mode. Here, we specified two lines and two columns. To make this matrix a diagram we just add directed arrows using the \ar command.

 ```\xymatrix{ A \ar[r] & B \ar[d] \\ D \ar[u] & C \ar[l] } ```

The arrow command is placed on the origin cell for the arrow. The arguments are the direction the arrow should point to (up, down, right, and left).

 ```\xymatrix{ A \ar[d] \ar[dr] \ar[r] & B \\ D & C } ```

To make diagonals, just use more than one direction. You can repeat directions to make bigger arrows.

 ```\xymatrix{ A \ar[d] \ar[dr] \ar[drr] & & \\ B & C & D } ```

We can draw even more interesting diagrams by adding labels to the arrows. To do this, we use the common superscript and subscript operators.

 ```\xymatrix{ A \ar[r]^f \ar[d]_g & B \ar[d]^{g'} \\ D \ar[r]_{f'} & C } ```

As shown, you use these operators as in math mode. The only difference is that that superscript means "on top of the arrow", and subscript means "under the arrow". There is a third operator, the vertical bar: | It causes text to be placed in the arrow.

 ```\xymatrix{ A \ar[r]|f \ar[d]|g & B \ar[d]|{g'} \\ D \ar[r]|{f'} & C } ```

To draw an arrow with a hole in it, use \ar[...]|\hole. In some situations, it is important to distinguish between different types of arrows. This can be done by putting labels on them or changing their appearance

 ```\xymatrix{ \bullet\ar@{->}[rr] && \bullet\\ \bullet\ar@{.<}[rr] && \bullet\\ \bullet\ar@{~)}[rr] && \bullet\\ \bullet\ar@{=(}[rr] && \bullet\\ \bullet\ar@{~/}[rr] && \bullet\\ \bullet\ar@{^{(}->}[rr] && \bullet\\ \bullet\ar@2{->}[rr] && \bullet\\ \bullet\ar@3{->}[rr] && \bullet\\ \bullet\ar@{=+}[rr] && \bullet } ```

Notice the difference between the following two diagrams:

 ```\xymatrix{ \bullet \ar[r] \ar@{.>}[r] & \bullet } ```
 ```\xymatrix{ \bullet \ar@/^/[r] \ar@/_/@{.>}[r] & \bullet } ```

The modifiers between the slashes define how the curves are drawn. Xy-pic offers many ways to influence the drawing of curves; for more information, check the Xy-pic documentation.

If you are interested in a more thorough introduction then consult the Xy-pic Home Page, which contains links to several other tutorials as well as the reference documentation.