Which invariant do you preserve?
Heck, how do you use the mouse in general? Stupid LJ poll time!
Which hand do you use to control the mouse?
Which button on your mouse is the primary? (i.e., which is used to select text? Which corresponds to the nominal "left-click"?)
Which finger do you use to click the primary mouse button?
Which finger do you use to click the secondary mouse button (if any)?