In Linux X11, each mouse button is assigned a number.
How to find the X11 mouse button number?
xev in terminal, then click the button to see its number.
xev, just close its window.
Here's the numbering:
〔➤ Linux: How to Swap Mouse Buttons〕
The numbering also depends on OS and mouse driver.
For example, on Mac OS X 10.4.x, using a Microsoft 5-buttons mouse, the numbering changes depending on whether you have Microsoft's mouse driver (IntelliPoint) installed.
If you don't have IntelliPoint installed, then Mac OS X doesn't register the mouse wheel tilt left/right, and your Backward (4th) button is the same as right-button, and Forward (5th) button is 4th button.
Here's the numbering reported by emacs's
<mouse-1>= left button
<mouse-2>= middle button (pressing the scroll wheel)
<mouse-3>= right button
<wheel-up>= roll scroll wheel up
<wheel-down>= roll scroll wheel down
<mouse-3>= 4th button (aka backward button)
<mouse-4>= 5th button (aka forward button)