Getting the scroll wheel on a mouse to work in XWindows

On a fresh installation of XWindows, the mouse is often configured to be a
basic mouse without a scroll wheel. The simplest way to correct this is
generally to run the appropriate X configuration script (xf86config for
XFree86, xorgconfig for X.Org), but if you just want to manually change your
configuration file, then there's one classic line that is supposed to be
added to your X config file to make the scroll wheel work. It looks like
this:

Option "ZAxisMapping" "4 5"

(By the way, if you're not familiar with hand-configuring your X install, the
relevant config file is typically either /etc/X11/xfree86.conf for XFree86,
or /etc/X11/xorg.conf for X.Org.)

This line should be added in the mouse section. The snag is that it might not
work on its own; essentially, this line is saying that buttons 4 and 5 on the
mouse should act like the two directions of the scroll wheel. (The act of
rolling your mouse's scroll wheel up or down is actually read as button
presses by the computer.) Since most mice have two regular buttons and the
scroll wheel acts as a third button, that usually means button numbers 4 and
5 are assigned to actual scrolling of the scroll wheel. However, if your
mouse has more buttons, these numbers may be different, and even if your
mouse only has two normal buttons, the numbers may be perversely different
just to make things more "interesting". You may need to fiddle with these
numbers a bit. Also, setting the mouse protocol to "Auto" usually helps; even
if you have a PS/2 mouse, setting the protocol to PS/2 may cause XWindows to
fail to recognize the scroll wheel, but "Auto" usually recognizes the scroll
wheel.