Psychologist George A. Miller proposed this in his 1956 paper "The Magical Number Seven, Plus or Minus Two." The real point isn't the number 7 — it's **chunking**: grouping items into meaningful units reduces memory and processing load.
It's why we write phone numbers as "819-555-6423" instead of one long string, and split card numbers into groups of four. In UI, this becomes splitting long forms into sections and grouping long lists by category.
A has to be scanned digit by digit; B is recognised as three chunks at once.
When to use
When laying out long numbers, forms, or lists. Don't chase the literal number 7 — focus on grouping by meaning.