The secondary selection is another way of selecting text using X. It does not use point or the mark, so you can use it to kill text without setting point or the mark.
mouse-set-secondary). The highlighting appears and changes as
you drag.
If you move the mouse off the top or bottom of the window while
dragging, the window scrolls at a steady rate until you move the mouse
back into the window. This way, you can mark regions that don't fit
entirely on the screen.
mouse-start-secondary).
mouse-secondary-save-then-kill). A second click
at the same place kills the secondary selection just made.
mouse-yank-secondary). This places point at the end of the
yanked text.
Double or triple clicking of M-Mouse-1 operates on words and lines, much like Mouse-1.
If mouse-yank-at-point is non-nil, M-Mouse-2
yanks at point. Then it does not matter precisely where you click; all
that matters is which window you click on. See section Mouse Commands for Editing.
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