GNU Emacs Manual
P.3 Using the Clipboard
As well as the primary and secondary selection types, X supports a
clipboard selection type which is used by some applications,
particularly under OpenWindows and Gnome.
The command M-x menu-bar-enable-clipboard makes the
Copy menu items, as well as the keys of the same
names, all use the clipboard.
You can customize the option
x-select-enable-clipboard to make
the Emacs yank functions consult the clipboard before the primary
selection, and to make the kill functions to store in the clipboard as
well as the primary selection. Otherwise they do not access the
clipboard at all. Using the clipboard is the default on MS-Windows,
unlike most systems.