keyboard copy paste

[Harry Putnam]

How can I copy and paste with the keybard between application windows.
I think I remember knowing how to do this at some time in the past but
now just coming up blank as to how.

I want to copy something in Emacs... ok .. no problem.. there are many
ways with keyboard inside emacs.

Now I want to paste whats on the clipboard into an Xterm cmd line,
without going to the mouse.

I'm running recent Xfce4 desktop... but not finding a way to do this.

Can I just steal whatever events happen when middle click on three
button mouse and put them into a keyboard shortcut?

How might I do that?