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The function key (often labelled Fn on the key) is usually used to switch on a simulated numeric keyboard, which is provided as a separate keypad on desktop keyboards. For those who don't want to use the simulation there are additional external numeric keypads available for PS/2 ports and I suppose USB ports. Also the function key may be used in combination with some F-keys to change display brightness, adjust the speaker volume or mute them, lock the keyboard, switch between external and internal display, use different suspend modes and more. Sometimes these key combinations work out of the box with Linux. Some require dedicated tools, for these tools see the Hotkey chapter above.