4. XFree 3.x

In XFree 3.x, additional pointing devices, like mice or tablets, are defined through the use of an Xinput section. The primary mouse is defined as usual:

Section "Pointer"
    Protocol    "PS/2"
    Device      "/dev/mouse"
In the case above, it's a regular three-button PS/2 mouse, which works for the trackpoint as well. Now, for the USB mouse:
    Section "Xinput"
  SubSection "Mouse"
    DeviceName "USB Mice"
    Protocol "IMPS/2"
    ZAxisMapping 4 5
    Port "/dev/input/mice"
My mouse is a wheel mouse, thus the protocol is IMPS/2. ZAxisMapping line is also needed for the wheel to work. I also want the mouse to always work alongside the other one, and this is achieved through the AlwaysCore option. If. for example, this was a serial mouse, the Port would read "/dev/ttyS0" for COM1.