Fn, or Function, is a modifier key on many keyboards, especially on laptops, used in a compact layout to combine keys which are usually kept separate. It is typically found on laptops, since a full sized keyboard would be difficult to fit in a laptop chassis. It is also found in many full-sized 'multimedia' and 'office' keyboards, named 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 appropriate key 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 or different colour (usually blue).

The Fn key is usually placed either to the right or to the left of the left Control key. In the latter case, it displaces the control key from the lower left corner of the keyboard. This is where the control key sits on keyboards without the Fn key. Many users, as well as professional laptop reviewers, prefer having the control key at the left end of the lower row.