666 (six hundred [and] sixty-six) is the natural number following 665 and preceding 667.

In Christianity, 666 is referred to in (most manuscripts of) chapter 13 of the Book of Revelation of the New Testament as the "number of the beast."^[1][2][3]

666 is the sum of the first thirty-six natural numbers, which makes it a triangular number:^[4]

${\displaystyle \sum _{i=1}^{36}i=1+2+3+\cdots +34+35+36=666}$.

Since 36 is also triangular, 666 is a doubly triangular number.^[5] Also, 36 = 15 + 21 where 15 and 21 are triangular as well, whose squares (15² = 225 and 21² = 441) add to 666 and have a difference of 216 = 6 × 6 × 6.

The number of integers which are relatively prime to 666 is also 216, ${\displaystyle \phi (666)=216}$;^[6] and for an angle measured in degrees, ${\displaystyle \sin {(666^{\circ })}=\cos {(216^{\circ })}=-{\tfrac {\varphi }{2}}}$ (where here ${\displaystyle \varphi }$ is the golden ratio).^{[7][8][lower-alpha 1]}

666 is also the sum of the squares of the first seven primes (2² + 3² + 5² + 7² + 11² + 13² + 17²),^[7][10] while the number of twin primes less than 6⁶ + 666 is 666.^[11]

A prime reciprocal magic square based on ${\displaystyle {\tfrac {1}{149}}}$ in decimal has a magic constant of 666. The twelfth pair of twin primes is (149, 151),^[12] with 151 the thirty-sixth prime number.^{[lower-alpha 2]}

666 is a Smith number and Harshad number in base ten.^[13][14] The 27th indexed unique prime in decimal features a "666" in the middle of its sequence of digits.^{[15][lower-alpha 3]}

The Roman numeral for 666, DCLXVI, has exactly one occurrence of all symbols whose value is less than 1000 in decreasing order (D = 500, C = 100, L = 50, X = 10, V = 5, I = 1).^[7]

• The Number of the Beast, the 1982 album by English heavy metal band Iron Maiden, references 666 in its title and the album's title song.
• Is the magic sum, or sum of the magic constants of a six by six magic square, any row or column of which adds up to 111.
• Is the sum of all the numbers on a roulette wheel (0 through 36).^[19] This is a corollary of the fact that the number is a Triangular number, as mentioned earlier.
• Was a winning lottery number in the 1980 Pennsylvania Lottery scandal, in which equipment was tampered to favor a 4 or 6 as each of the three individual random digits.^[21]
• Was the original name of the Macintosh SevenDust computer virus that was discovered in 1998.^{[citation needed]} It is also the name of an extension that SevenDust can add to an uninfected Macintosh.^[22]
• The number is a frequent visual element of Aryan Brotherhood tattoos.^[23]
• Aleister Crowley adopted the title "the Beast 666".^[24] As such, 666 is also associated with him, his work, and his religious philosophy of Thelema.
• Molar mass of the high-temperature superconductor YBa₂Cu₃O₇.
• In Chinese numerology, the number is considered to be lucky and is often displayed in shop windows and neon signs.^[25][26] In China, 666 can mean "everything goes smoothly" (the number six has the same pronunciation as the character 溜, which means "smooth".^[27]
• Is commonly used by ISPs to blackhole traffic using BGP communities.^[28]
• 666 Fifth Avenue in New York City, which was bought for $1.8 billion in 2007, was the most expensive real estate deal in New York's history.^[29]

• Numerology