31 (thirty-one) is the natural number following 30 and preceding 32. It is a prime number.
31 is a prime number, a twin prime (with 29), and a Mersenne prime.
31 equal temperament 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.
31 is the international calling code for the Netherlands.