Change mouse button sequence

Normally computer mouse function in such a way that the first button is pressed using the index finger of the right hand. People who are left-handed frequently find that it is more comfortable to reverse the button codes that get generated so that the primary button is pressed using the index finger of the left hand.

To do this:
Go to System >Preferences > Mouse and select 'Left-handed' from 'Mouse orientation'.

If you want to do some naughty things with mouse keys like to keep first button in second position of mouse or to disable particular mouse key use the command line program 'xmodmap'.It is there in default Ubuntu installation.

Open Applications > Accessories > Terminal

To get current mouse button sequence, run
xmodmap -pp
My xmodmap -pp gave me a list of 32 defined pointer buttons!Out of which maximum 4 we use,if we have a 4 button mouse.

Depending upon your mouse,
To change the sequence, run
xmodmap -e "pointer = 3 2 1"
where "3 2 1" is the changed position from default "1 2 3".If you want other combinations, try yourself.

To disable particular key, say first physical button, run
xmodmap -e "pointer = 9 3 1"
Here I have assigned mouse button "9" (which is not functional) to first physical position.