A commutative diagram with exact rows, showing the naturality of the snake lemma. Produced with TeX and XY-Pic, using the source code below. Both the image and the source code are in the public domain.

% Produce a postscript file from this input with tex and dvips.

% Loading the XY-Pic package:
\input xy
\xyoption{all}

% Using postscript driver for smoother curves
\xyoption{ps}
\xyoption{dvips}

% No need for page numbers
\nopagenumbers

\def\coker{\mathop{\rm coker}}

$$
\xymatrix{ %Our diagram is a 2x7 matrix
&{}\ker a_2\ar[r]&{}\ker b_2\ar[r]&{}\ker c_2\ar[r]^{d_2}&{}\coker a_2\ar[r]&{}\coker
b_2\ar[r]&{}\coker c_2\\
{}\ker a_1\ar[r]\ar[ur]&{}\ker b_1\ar[r]\ar[ur]&{}\ker c_1\ar[r]^{d_1}\ar[ur]&{}\coker 
a_1\ar[r]\ar[ur]&{}\coker b_1\ar[r]\ar[ur]&{}\coker c_1\ar[ur]
}
$$

\end