In addition to the primary selection, the X Window System provides a second similar facility known as the secondary selection. Nowadays, few X applications make use of the secondary selection, but you can access it using the following Emacs commands:
Set the secondary selection, with one end at the place where you press
down the button, and the other end at the place where you release it
(mouse-set-secondary). The selected text is highlighted, using
the secondary-selection face, as you drag. The window scrolls
automatically if you drag the mouse off the top or bottom of the
window, just like mouse-set-region (see Mouse Commands for Editing).
This command does not alter the kill ring.
Set one endpoint for the secondary selection
(mouse-start-secondary); use M-mouse-3 to set the other
end and complete the selection. This command cancels any existing
secondary selection, when it starts a new one.
Set the secondary selection (mouse-secondary-save-then-kill),
with one end at the position you click M-mouse-3, and the other
at the position specified previously with M-mouse-1. This also
puts the selected text in the kill ring. A second M-mouse-3 at
the same place kills the text selected by the secondary selection just
made.
Insert the secondary selection where you click, placing point at the
end of the yanked text (mouse-yank-secondary).
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, or even
which of the frame’s windows you click on. See Mouse Commands for Editing.
This user option also effects interactive search: if it is
non-nil, yanking with the mouse anywhere in the frame will add
the text to the search string.