The number 7

Established fact In maths and on the calendar

• 7 is prime: its only divisors are 1 and 7.
• 7 is a side of the 7-24-25 right triangle (7² + 24² = 25²).
• In 1956 the psychologist George Miller described short-term memory as holding about seven items, give or take two; later research puts it nearer four meaningful chunks.
• Every month has a 7th, so it comes round twelve times a year.