2

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Cardinal	two
Ordinal	2nd (second / twoth)
Numeral system	binary
Factorization	prime
Gaussian integer factorization	${\displaystyle (1+i)(1-i)}$
Prime	1st
Divisors	1, 2
Greek numeral	Β´
Roman numeral	II, ii
Greek prefix	di-
Latin prefix	duo-/bi-
Old English prefix	twi-
Binary	102
Ternary	23
Senary	26
Octal	28
Duodecimal	212
Hexadecimal	216
Greek numeral	β'
Arabic, Kurdish, Persian, Sindhi, Urdu	٢
Ge'ez	፪
Bengali	২
Chinese numeral	二，弍，貳
Devanāgarī	२
Telugu	౨
Tamil	௨
Kannada	೨
Hebrew	ב
Armenian	Բ
Khmer	២
Maya numerals	••
Thai	๒
Georgian	Ⴁ/ⴁ/ბ(Bani)
Malayalam	൨
Babylonian numeral	𒐖
Egyptian hieroglyph, Aegean numeral, Chinese counting rod	||
Morse code	.._ _ _

2 (two) is a number, numeral and digit. It is the natural number following 1 and preceding 3. It is the smallest and the only even prime number.

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

Two is most commonly a determiner used with plural countable nouns, as in two days or I'll take these two.^[1] Two is a noun when it refers to the number two as in two plus two is four.

The word two is derived from the Old English words twā (feminine), tū (neuter), and twēġen (masculine, which survives today in the form twain).^[2]

The pronunciation /tuː/, like that of who is due to the labialization of the vowel by the w, which then disappeared before the related sound. The successive stages of pronunciation for the Old English twā would thus be /twɑː/, /twɔː/, /twoː/, /twuː/, and finally /tuː/.^[2]

The number two has several mathematical properties and applications. 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.^[3] 2 is the smallest and the only even prime number, and the first Ramanujan prime.^[4]

It is the smallest cardinal number required to form a binary system, which is the basis of most machine languages. In this system, natural numbers are represented using only two symbols (0 and 1), making it the simplest consistent numbering system.

In mathematics, two is notable for being the smallest number that makes the Mertens function return to zero. It is also the Euler characteristic of polyhedra.

In the context of logarithmic calculations, two serves as the base of the binary logarithm.

Regarding factorial values, both numbers one and two are unique because their factorials are equal to themselves. Additionally, two, along with one and zero, are the only numbers that are Harshad numbers in every number base. Harshad numbers are integers that are divisible by the sum of their digits when expressed in any base.

The number two holds several distinctions in various mathematical sequences and concepts:

Prime Number: Two is the first and only even prime number, followed by three. Eisenstein Prime: It is the first Eisenstein prime without an imaginary part, succeeded by five. Sophie Germain Prime: Two is the first Sophie Germain prime, followed by three. Stern Prime: It is the first in the Stern prime sequence, followed by three. Factorial Prime: Two is the smallest factorial prime, followed by three. Lucas Number: It is the first Lucas number, succeeded by one. Perrin Prime: It appears twice in the Perrin prime sequence, as the third and fifth elements, both times followed by three. Smarandache-Wellin Number: It is the first Smarandache-Wellin number, succeeded by twenty-three. Primorial: Two is the smallest primorial, succeeded by six. Sequence Term: It is the first term of the sequence (1 + 1/n)^n, which approaches the limit of the number e. It also holds positions in other mathematical sequences:

Markov Number: Two is the second Markov number, preceded by one and followed by five. Motzkin Number: It is the second Motzkin number, after one and before four. Pell Number: Two is listed as the second and a third Pell number, coming after one and before five in its initial appearance. Additional classifications:

Bell Number: It is the third Bell number, preceded by one and succeeded by five. Fibonacci Sequence: Two is the third term in the Fibonacci sequence, following one and preceding three.

A digon is a polygon with two sides (or edges) and two vertices.^[5]: 52 Two distinct points in a plane are always sufficient to define a unique line in a nontrivial Euclidean space.^[6]

A set that is a field has a minimum of two elements.^{[citation needed]} A Cantor space is a topological space ${\displaystyle 2^{\mathbb {N} }}$ homeomorphic to the Cantor set.^{[citation needed]}

Binary is a number system with a base of two, it is used extensively in computing.^[7]

The digit used in the modern Western world to represent the number 2 traces its roots back to the Indic Brahmic script, where "2" was written as two horizontal lines. The modern Chinese and Japanese languages (and Korean Hanja) still use this method. The Gupta script rotated the two lines 45 degrees, making them diagonal. The top line was sometimes also shortened and had its bottom end curve towards the center of the bottom line. In the Nagari script, the top line was written more like a curve connecting to the bottom line. In the Arabic Ghubar writing, the bottom line was completely vertical, and the digit looked like a dotless closing question mark. Restoring the bottom line to its original horizontal position, but keeping the top line as a curve that connects to the bottom line leads to our modern digit.^[8]

In fonts with text figures, digit 2 usually is of x-height, for example, .^{[citation needed]}

• The number of polynucleotide strands in a DNA double helix.^[9]
• The first magic number.^[10]
• The atomic number of helium.^[11]

• Binary number

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