Thirty-one

For the year A.D. 31, see 31.

Thirty-one (31, XXXI) is the natural number following thirty and preceeding thirty-two. It is a prime number, the third Mersenne prime as well as a primorial prime, and together with twenty-nine, another primorial prime, it comprises a twin prime.

Thirty-one is also:

The atomic number of gallium.
The number of days in the months January, March, May, July, August, October and December.
The code for international direct dial phone calls to Netherlands.
A type of card game. See Thirty-one (game).
The number of flavors of Baskin Robbins.
In Star Trek, the name of Section 31, a secret police shadow organization of the Federation.

List of numbers

Twenty-nine - Thirty - Thirty-one - Thirty-two - Thirty-three