Editing Fitts's law (section)

== Implications for UI design ==
[[File:Magic Corners English.jpg|thumb|Magic corners in [[Microsoft Windows]]]]
[[File:Radial Menu English.jpg|thumb|[[Radial menu]]]]

Multiple design guidelines for [[GUI]]s can be derived from the implications of Fitts's law. In its basic form, Fitts's law says that targets a user has to hit should be as big as possible. This is derived from the ''W'' parameter. More specifically, the effective size of the button should be as big as possible, meaning that its form has to be optimized for the direction of the user's movement onto the target.

Layouts should also cluster functions that are commonly used with each other. Optimizing for the ''D'' parameter in this way allows for smaller travel times.

Placing layout elements on the four edges of the screen allows for infinitely large targets in one dimension and therefore presents ideal scenarios. Since the pointer will always stop at the edge, the user can move the mouse with the greatest possible speed and still hit the target. The target area is effectively infinitely long along the movement axis. Therefore, this guideline is called “Rule of the infinite edges”. The use of this rule can be seen for example in [[MacOS]], which always places the menu bar on the top left edge of the screen instead of the current program's windowframe.<ref name="Hale2007">{{cite web |url=http://www.particletree.com/features/visualizing-fittss-law/ |title=Visualizing Fitts's Law |last=Hale |first=K |publisher=Particletree |date=2007 |url-status=live |archive-url=https://archive.today/20191208221824/http://www.particletree.com/features/visualizing-fittss-law/ |archive-date=2019-12-08 |access-date=2019-12-08 }}</ref>

This effect can be exaggerated at the four corners of a screen. At these points two edges collide and form a theoretically infinitely big button. [[Microsoft Windows]] (prior to [[Windows 11]]) places its "Start" button in the lower left corner and [[Microsoft Office]] 2007 uses the upper left corner for its "Office" menu. These four spots are sometimes called "magic corners".<ref name="Jensen2002">{{cite web |url=https://blogs.msdn.microsoft.com/jensenh/2006/08/22/giving-you-fitts/ |title=Giving You Fitts |last=Jensen |first=H. |publisher=Microsoft Developer |date=2006 |url-status=live |archive-url=https://archive.today/20191208222139/https://blogs.msdn.microsoft.com/jensenh/2006/08/22/giving-you-fitts/ |archive-date=2019-12-08 |access-date=2019-12-08 }}</ref>
[[MacOS]] places the close button on the upper left side of the program window and the menu bar fills out the magic corner with another button.

A UI that allows for pop-up menus rather than fixed drop-down menus reduces travel times for the ''D'' parameter. The user can continue interaction right from the current mouse position and doesn't have to move to a different preset area. Many operating systems use this when displaying right-click context menus. As the menu starts right on the pixel which the user clicked, this pixel is referred to as the "magic" or "prime pixel".<ref name="Zhao2002"/>

James Boritz et al. (1991)<ref name="Boritz1991">{{cite journal |title=Fitts's law studies of directional mouse movement |journal=Human Performance |year=1991 |last1=Boritz |first1=J |last2=Cowan |first2=W. B. |s2cid=43676399 |volume=1 |issue=6 }}</ref> compared [[radial menu]] designs. In a radial menu all items have the same distance from the prime pixel. The research suggests that in practical implementations the direction in which a user has to move their mouse has also to be accounted for. For right-handed users, selecting the left-most menu item was significantly more difficult than the right-most one. No differences were found for transitions from upper to lower functions and vice versa.