Fitts's law
Fitts’s law states that the time to click on something depends on distance and size. Rauno Freiberg’s gloss in Invisible Details of Interaction Design is that the bigger the target, and the closer 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; Rauno also lists Kevin Hale’s 2010 Visualizing Fitts’s Law.
Figure 3 lets you turn the two variables. Change distance and width, press Move, and watch difficulty rise as the target shrinks or moves away. Then turn on the edge target and notice the width reads as unbounded. Finally switch on the radial menu, where six wedges spawn around the pointer. The two switches are Rauno’s two examples.
Magic corners are infinite targets#
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 any corner becomes a quick flick of the mouse. That is why commonly used menus, like the Apple menu, sit in corners. The target’s size in pixels did not change; its effective size did.
Radial menus spawn at the pointer#
Radial menus spawn around the pointer, making the size of and distance to every target the same for every action. Nothing is far, small, or privileged by position. Over time muscle memory kicks in and you can select purely by distance and direction, without reading labels.
His own demo, published separately as the Radial Menu prototype (January 2023), carries the instruction “Hold and rotate from anywhere”: the whole affordance in four words. The menu does not have a place; it appears where you are.
Three levers, no numbers#
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 decided before any animation is chosen, and the radial menu is where they meet: a menu that spawns where you are has nothing to travel, so the only motion left is your hand’s.
The law gives you three levers and nothing to memorise. Make frequent targets bigger. Put them where the pointer will be when the need arises, near the trigger or at an edge it cannot pass. And when an action has several equal options, consider spawning them around the pointer, so distance and direction can become the interface once the hand has learned them.