Intro.

There are four buttons on the Kensington Slimblade trackball mouse. I wanted to use the left-top button as Backward button and the right-top one as Forward. There would be some other solutions for mouse button mapping, but I used evdev which operates on a X11 layer.

Solution

Add following lines to the file: /usr/share/X11/xorg.conf.d/10-evdev.conf.

# Kensington Slimblade Settings

Section "InputClass"
Identifier "Kensington Slimblade Trackball"
MatchProduct "Kensington Slimblade Trackball"
MatchIsPointer "on"
MatchDevicePath "/dev/input/event*"
Driver "evdev"

Option "ButtonMapping" "1 8 3 4 5 6 7 9"

EndSection

The key line is Option "ButtonMapping" "1 8 3 4 5 6 7 9 2". The final string indecates that it will map the button 1 to 1, 2 to 8, 3 to 3, and so on. (The position number to the described number).

Use xev tool to figure out the number of each button on your mouse.

In my case, I was helped with the other mouse to get the number of a Backward button and a Forward button. Those were 8 and 9 respectively.

Finally, I could map the left-top button(2) to Backward button(8) and the right-top button(8) to Forward button(9).