Fitts' Law
The time to acquire a target is a function of the distance to and size of the target.
Size
Touch targets should be large enough for users to accurately select.
Spacing
Touch targets should have ample spacing between them.
Placement
Touch targets should be placed in areas of an interface that allow them to be easily accessed.
Origin
In 1954, psychologist Paul Fitts, examining the human motor system, showed that the time required to move to a target depends on the distance to it, yet relates inversely to its size. By his law, fast movements and small targets result in greater error rates, due to the speed-accuracy trade-off. Although multiple variants of Fitts’ law exist, all encompass this idea. Fitts’ law is widely applied in design. For example, this law influenced the convention of making interactive buttons large (especially on finger-operated mobile devices); smaller buttons are more difficult (and time consuming) to click. Likewise, the distance between a user’s task/attention area and the task-related button should be kept as short as possible.