Fitts's law

Fitts’s law states that the time to click on something depends on distance and size. Rauno Freiberg’s one-line gloss in Invisible Details of Interaction Design is that the bigger the target, and the closer it is to where your pointer already is, the better. The law comes from Paul Fitts’s 1954 paper on the information capacity of the human motor system, and Rauno also lists Kevin Hale’s 2010 article Visualizing Fitts’s Law among his resources. It is one of the few observations in the essay that arrives with a paper attached.

Figure 3 lets you turn the two variables. Change the distance and the width, press Move, and watch the difficulty rise as the target shrinks or moves away. Then turn on the edge target and notice that the width reads as unbounded. Finally, switch on the radial menu, where six wedges spawn around the pointer instead of waiting somewhere on the stage. The two switches correspond to the two examples Rauno gives.

The first example is the screen edge. Operating systems use magic corners because the target area there is infinitely large: the pointer cannot overshoot past the corner, so the precision required is very low and reaching for any corner becomes a quick flick of the mouse. That, he says, is why operating systems place commonly used menus, like the Apple menu, in corners. That is what the figure meant by an unbounded width: the target’s size in pixels did not change, its effective size did.

The second example is the radial menu. Radial menus spawn around the pointer, which makes the size of and distance to every target the same for every action. Nothing is far, nothing is small, and nothing is privileged by position. Rauno adds a second effect that shows up with use: over time, muscle memory kicks in and it becomes possible to select an action purely by distance and direction, without reading the labels. His own demo, embedded in the essay and published separately as the Radial Menu prototype (January 2023), carries the instruction “Hold and rotate from anywhere,” which is the whole affordance in four words. The menu does not have a place; it appears where you are.

Notice how this connects to the earlier sections. The App Switcher and the Dynamic Island are about where things come from; Fitts’s law is about where they should be when you need them. Both are spatial arguments, and both are decided before any animation is chosen. Rauno’s own radial menu is where the two meet: a menu that spawns where you are has nothing to travel, so there is nothing to animate across the screen, and the only motion left is your hand’s.

For your own interfaces the law gives you three levers and no numbers to memorise. Make frequent targets bigger. Put them closer to where the pointer will be when the need arises, which on the desktop means near the trigger or at an edge the pointer cannot pass. And when an action has several equal options, consider spawning them around the pointer rather than laying them out in a row somewhere else, so that distance and direction can become the interface once the hand has learned them.