The mouse commands for selecting and copying a region are mostly
compatible with the xterm program. You can use the same mouse
commands for copying between Emacs and other X client programs.
mouse-set-point).
This is normally the left button.
mouse-set-region). You can specify both ends of
the region with this single command.
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 select regions that don't fit
entirely on the screen.
mouse-yank-at-click).
This is normally the middle button.
mouse-save-then-kill, has several functions
depending on where you click and the status of the region.
The most basic case is when you click Mouse-1 in one place and
then Mouse-3 in another. This selects the text between those two
positions as the region. It also copies the new region to the kill
ring, so that you can copy it to someplace else.
If you click Mouse-1, scroll with the scroll bar, and then click
Mouse-3, it remembers where point was before scrolling (where you
put it with Mouse-1), and uses that position as the other end of
the region. This is so that you can select a region that doesn't fit
entirely on the screen.
More generally, if you do not have a highlighted region, Mouse-3
selects the text between point and the click position as the region. It
does this by setting the mark where point was, and moving point to where
you click.
If you have a highlighted region, or if the region was set just before
by dragging button 1, Mouse-3 adjusts the nearer end of the region
by moving it to where you click. The adjusted region's text also
replaces the old region's text in the kill ring.
If you originally specified the region using a double or triple
Mouse-1, so that the region is defined to consist of entire words
or lines, then adjusting the region with Mouse-3 also proceeds by
entire words or lines.
If you use Mouse-3 a second time consecutively, at the same place,
that kills the region already selected.
The simplest way to kill text with the mouse is to press Mouse-1 at one end, then press Mouse-3 twice at the other end. See section Deletion and Killing. To copy the text into the kill ring without deleting it from the buffer, press Mouse-3 just once--or just drag across the text with Mouse-1. Then you can copy it elsewhere by yanking it.
To yank the killed or copied text somewhere else, move the mouse there
and press Mouse-2. See section Yanking. However, if
mouse-yank-at-point is non-nil, Mouse-2 yanks at
point. Then it does not matter precisely where you click; all that
matters is which window you click on. The default value is nil.
This variable also effects yanking the secondary selection.
To copy text to another X window, kill it or save it in the kill ring. Under X, this also sets the primary selection. Then use the "paste" or "yank" command of the program operating the other window to insert the text from the selection.
To copy text from another X window, use the "cut" or "copy" command of the program operating the other window, to select the text you want. Then yank it in Emacs with C-y or Mouse-2.
When Emacs puts text into the kill ring, or rotates text to the front
of the kill ring, it sets the primary selection in the X server.
This is how other X clients can access the text. Emacs also stores the
text in the cut buffer, but only if the text is short enough
(x-cut-buffer-max specifies the maximum number of characters);
putting long strings in the cut buffer can be slow.
The commands to yank the first entry in the kill ring actually check first for a primary selection in another program; after that, they check for text in the cut buffer. If neither of those sources provides text to yank, the kill ring contents are used.