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Definition: Fn key

(FuNction key) A keyboard modifier key that works like a Shift key to activate a second function on a dual-purpose key. Commonly found on laptop keyboards, the Fn key is used to control hardware functions such as screen brightness and speaker volume. It may also be found on extra small keyboards for desktop computers, typically for Page Up and Page Down, which are combined with Up Arrow and Down Arrow. See function key and computer keyboard.

Like the Shift Key
The Fn key activates functions on dual-purpose keys, which in this example are F11 and F12. When Fn is held down and F11 and F12 are pressed, F11 lowers speaker volume, and F12 raises it.