Fitts' Law

Fitts' Law is a rule of psychomotor function which states that the time required to move to a target is a function of the size of the target and the distance to it. It is formally stated as $$MT = a + b \log_{2}(\frac {2A} {W} )$$, where MT is the movement time, a and b are regression coefficients, A is the distance to move to the center of the target, and W is the width of the target. The law is named for Paul Fitts, who first stated it in 1954.

Fitts' Law is of particular value in designing graphic user interfaces. Simply put, it states that the bigger and closer a target, the easier it is to move to. In the case of a typical graphical display, where the cursor is forced to stop moving at the edge of the screen, any element which touches the edge of the screen can be considered to have near-infinite size in the dimension which reaches the screen edge.

This leads to the usability rule of the "five magic pixels": the five places on the screen a user can move a pointer to the most easily are the four corners of the screen, since they have near-infinite size in both dimensions, and the spot where the pointer already is.