Fitts's law

Paul Fitts published the model in 1954, about people moving a stylus between metal plates, and it has survived every input device invented since. The claim is that pointing is a trade of speed against accuracy, and that the difficulty of a move can be written as a single number: the index of difficulty, roughly the base-two logarithm of twice the distance divided by the width of the target. Movement time is then a straight line in that number, with two constants that belong to the device and the person rather than to the interface. The units are bits, because the model is really about how much information the hand has to resolve to land on the thing.

Two consequences are worth carrying around, and they are not symmetric. Distance is inside a logarithm, so halving it saves you less than you would guess, while width is also inside the logarithm but is the term you usually have the most freedom over. That is why “make it bigger” beats “move it closer” as advice, and why a control used constantly earns its extra pixels. It also explains something people notice without naming: a small target you have to travel to is disproportionately annoying, because the two penalties compound inside the same expression.

The famous corollary is about edges. A pointer stops at the boundary of the screen, so a control flush against an edge behaves as though it extends past it forever: the effective width in the direction of travel is unbounded, the index of difficulty collapses, and you can throw the pointer at it without aiming. A corner is two edges at once, which is why the Start button and the Apple menu sit where they do, and why the macOS menu bar being welded to the top of the screen has always been faster to hit than a menu bar inside a window a few pixels below it. The web mostly forfeits this: a browser page has margins, and a sticky bar with padding around it is an edge target that is not touching the edge.

The law is a model of aimed movement, not a law of usability, and it says nothing about whether people can find the control or understand it. It also assumes an approach from a known starting point, which is why the answer to “where should this button go” is usually “near whatever the reader was just doing” rather than “in the middle”. On touch there is no travel to measure at all, and the size half of the trade takes over completely, which is where target size and its minimums come back into it.