How does copy-paste work with xterm?

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http://askubuntu.com – Left-down-clicking in xterms starts the selection of something to be copy-pasted. Double-left clicking selects a word. Triple-left clicking selects a line. All this works under unity in 11.04. However, there is no way to copy-paste that selection to another place: The right-click menu shows paste disabled, and middle-clicking to copy-paste does not work. So how can I copy a selection from an xterm into another place? (I am using the default-installation no special configuration so far) (HowTos)