31 (number)

31 is the natural number following 30 and preceding 32. It is a prime number.

Mathematics

31 is a prime number, a twin prime, and a Mersenne prime.

In other fields

is a popular microtonal tuning for musical instruments because it provides a good approximation of harmonic intervals.
January, March, May, July, August, October and December have 31 days.
California was the 31st state to join the United States.
31 is the international calling code for the Netherlands.