The mouse button number verses left, right middle button highlights one of the limitations with adopting 'popular' terminology. Many of these other applications have limited facilities for binding mouse buttons. Often, if you have a mouse with more than 3 buttons, you can't use them or you have to go to weird nonintuitive lengths to use them. Emacs does not suffer from such a limitation. and the binding mechanism is the same regardless of the button. Using left, middle and right is in fact a very poor choice because it falls down as soon as you have a mouse with a different form factor, use the mouse in the left hand instead of right or use a device like a touch pad, foot peddle/pad or one of the many adaptive technology devices used by someone with a disability. meanwhile, mouse-1, mouse-2, mouse-n remain clear and unambigious.