The Magical Number Seven, Plus or Minus Two: Some Limits on Our Capacity for Processing Information is a 1956 paper by the cognitive psychologist George A. Miller. In it Miller showed a number of remarkable coincidences between the channel capacity of a number of human cognitive and perceptual tasks. In each case, the effective channel capacity is equivalent to between 5 and 9 equally-weighted error-less choices: on average, about 2.5 bits of information. Miller hypothesized that these may all be due to some common but unknown underlying mechanism.

The concept of this limit is illustrated by imagining the patterns on the faces of a die. It is easy for many people to visualise each of the six faces. Now imagine seven dots, eight dots, nine dots, ten dots, and so on. At some point it becomes impossible to visualise the dots as a single pattern (a process known as subitising), and one thinks of, say, eight as two groups of four. The upper limit of your visualisation of a number represented as dots is your subitising limit for that exercise.