Notebook + External Display: How to tell Xubuntu not to clone but to expand the laptop display to the external one?

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http://askubuntu.com – I went to the Settingsmanager and selected Display. The external display was listed there. So I selected Use this output (see Screenshot) and after hitting apply the notebook screen was cloned to the external display. But what I wanted was for it to be expanded, not cloned. Unfortunately I cannot find any option for that. Is there such an option? Greatful for any hints! (HowTos)