xmodmap(1) - Linux man page: "Many pointers are designed such 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. This could be done on a 3 button pointer as follows:
% xmodmap -e "pointer = 3 2 1"
"
