Fitt's Law

The time to acquire a target is a function of the distance to and the size of the target.

-- Paul Fitts, 1954

Originating in psychology and usability studies, this law studied the ease of grasping a physical target. The law is of direct importance to user interface designers today, and to the software developers who create those interfaces. It is also applicable to software interfaces: for example, it manifests in the ease-of-use of APIs.