The Fn key, short form for function, is a modifier key on many keyboards, especially external keyboards, and is not available for mobile devices. For use in a compact layout, combine keys which are normally kept separate. Alt + Fn is typically found on laptops due to their keyboard size restrictions. It is also found on many full-sized "multimedia" keyboards as the F-Lock key. It is mainly for the purpose of changing display or audio settings quickly, such as brightness, contrast, or volume, and is held down in conjunction with the Caps Lock to change the settings.

Typically, in a compact layout the main area of the keyboard (containing the letter keys) is kept in much the same layout as with a full-sized keyboard, and the numeric keypad is moved to share a group of central keys. This allows typists to enter text without having to learn a new layout. The symbol that is accessed through pressing Fn is often printed on the key in a smaller font, a box, or different colour (usually blue but sometimes orange). ^{[citation needed]}

This 78-key UK layout is taken from the Apple iBook. Characters in blue are accessed through the Fn key, but functions on the top row can be accessed without Fn key in Mac OS X.

Fn is a modifier key, and works like other modifiers keys, such as Ctrl, Shift, Alt and AltGr. For a standard modifier key, the microcontroller inside the keyboard sends a scancode for the modifier itself, which is then interpreted by the operating system and combined with other simultaneous key-presses. The Fn key is a form of meta-modifier key, in that it causes the operating system to see altered scancodes when other keys on the keyboard are pressed. However, because the operating system has no notion of the Fn key, the key can not normally be remapped in software, unlike all other standard keyboard keys.

While it is more common for the Fn key processing to happen directly in the keyboard micro-controller, offering no knowledge to the main computer of whether the Fn key was pressed, some manufacturers, like Lenovo, perform this mapping in BIOS, allowing remapping the Fn key for the built-in keyboard; and Apple, in which the Fn key is mappable and serves other uses too, as triggering the Dictation function by pressing the Fn key twice, and changing the input source if there are multiple or showing the emoji and symbols menu by pressing the Fn key once.

In addition to being mapped to standard keyboard keys like Scroll Lock and Num Lock, the Fn key combinations may also be mapped to control system interfaces to change the LCD brightness, VGA output, or speaker volume on most laptop computers.

Microsoft Windows added native support for Fn keys since Windows 2000 and Windows Me.

There is not yet an agreed-upon standard for the placement of the Fn key, although most manufacturers have elected to place it alongside a shrunken and/or displaced left Control key. Because the Control key is most frequently associated with OS and application shortcuts (such as Control+S to save a document, or Control+Shift+Escape to launch the Task Manager in modern versions of Microsoft Windows), altering its size and placement is often regarded as inconvenient for users accustomed to the larger left Control key on IBM PC-style keyboards commonly used for desktop computers.