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I expect Jarno has heard about this already, but just to nitpick a bit: in X Window System, selecting a piece of text does not normally copy it to the clipboard, despite this being a widely-held belief. Or well, it does, but it's not the only clipboard, and in X parlance, it's usually called the PRIMARY selection. There are three standard selections: PRIMARY, SECONDARY and CLIPBOARD.

To copy things to "the" clipboard — the CLIPBOARD selection, that is — you usually have to invoke an explicit copy operation, typically via the menu choice Edit -> Copy (or your localized equivalent) or via a keyboard shortcut, typically Ctrl-C. You might notice some similarities to certain other systems. And the same goes for the opposite actions: middle clicking only pastes the PRIMARY selection. To paste the contents of the clipboard, Edit -> Paste or Ctrl-V is typically the way.

For more information, see the explanation at freedesktop.org.