12 is:

• A composite number: It has factors other than 1 and itself (i.e., 2, 3, 4, 6, 12).
• An even number: It is divisible by 2.
• A highly composite number: It has more divisors (6) than any smaller positive integer.
• A pronic number: It is the product of two consecutive integers (3 and 4).
• A triangular number: It can be represented as the sum of consecutive positive integers (1 + 2 + 3 + 4 + 5 + 6 = 12).
• A Catalan number: It counts the number of ways to parenthesize an expression with 4 variables.
• A Harshad number: It is divisible by the sum of its digits (1 + 2 = 3, and 12 is divisible by 3).
• A refactorable number: It can be written as the sum of two cubes in two different ways (2³ + 1³ = 9, and 1³ + 1³ + 1³ + 1³ = 12).
• The atomic number of magnesium: It has 12 protons in its nucleus.
• The number of months in a year: A year has 12 months.
• The number of hours in half a day: There are 12 hours from noon to midnight or from midnight to noon.