2

Because it forms the basis of a duality, it has religious and spiritual significance in many cultures.

Mathematics

The number 2 is the second natural number, after 1. Each natural number, including 2, is constructed by succession, that is, by adding 1 to the previous natural number. 2 is the smallest and the only even prime number, and the first Ramanujan prime. It is also the first superior highly composite number, and the first colossally abundant number.

An integer is determined to be even if it is divisible by two. When written in base 10, all multiples of 2 will end in 0, 2, 4, 6, or 8; more generally, in any even base, even numbers will end with an even digit.

Binary is a number system with a base of two, where each "bit" (binary digit) is either 0 (off) or 1 (on). It is used extensively in computing, since simple on-off logic is relatively simple to keep track of with electronics.

A digon is a polygon with two sides (or edges) and two vertices. In Euclidean space, digons are degenerate, collapsing to a line segment between the two vertices. In spherical geometry, however, non-degenerate digons can exist.

Two distinct points in a plane are always sufficient to define a unique line in a nontrivial Euclidean space.

The integers modulo 2 form the finite field ${\displaystyle {\mathbb{F}}_2}$, the smallest finite field. It has two elements, usually denoted 0 and 1, and addition in ${\displaystyle {\mathbb{F}}_2}$ corresponds to parity. Thus reduction modulo 2 records the parity of an integer: even integers are congruent to 0 modulo 2, and odd integers are congruent to 1 modulo 2. In algebra, structures of characteristic 2 have special behavior because ${\displaystyle 1+1=0}$; in particular, every element satisfies ${\displaystyle x=-x}$. For this reason, many algebraic constructions have separate forms in characteristic 2.

A symmetry of order two is called an involution.

List of basic calculations