Most users of X11 use a 3 button mouse, so they get a "middle click" (in addition to left click and right click). In recent years there's been a glut of 2-button mice. There are ways of using a click of both buttons together as a middle mouse button, but that is usually less convenient than having a real 3 button mouse. Also, some 3 button mice actually have the middle button disconnected!
In X11's standard Athena widget set (aka, apparently inaccurately, Xt), middle click is used for 2 important tasks:
- In the text widget, a middle click pastes the current selection. Note that no explicit action on the selection is required: just mark the region with a left click and copy it with a middle click.
- In the scrollbar, a middle click drags the slider to the current pointer location. You needn't catch hold of the slider, the entire scrollbar works for this (useful when the slider is just a few pixels). This is much better than what badly designed scrollbars offer you, despite being much less popular.