21

Twenty-one is the natural number that comes after twenty and before twenty-two. In mathematics, it is an odd composite number equal to 3 × 7 and has divisors 1, 3, 7, and 21. It is the sixth triangular number, since 1 + 2 + 3 + 4 + 5 + 6 = 21. It is also a semiprime and a Fibonacci number (F8). It is a Harshad number in base 10, because 21 is divisible by the sum of its digits (2 + 1 = 3).

In numerals, 21 is written as XXI in Roman numerals; in binary it is 10101, in hexadecimal 0x15. In other bases, it equals 210 in base 3 and 111 in base 4, among others.

The atomic number of scandium is 21. Scandium is a soft, light transition metal discovered in 1879 by Lars Fredrik Nilson and named for Scandinavia. It is used to strengthen aluminum alloys and finds applications in aerospace, sports equipment, and high-performance lighting.

In culture and society, 21 marks a milestone in many legal systems. In the United States, 21 is the minimum legal drinking age. The number is also associated with the card game blackjack, where a hand totaling 21 is the best possible score. A 21-gun salute is a ceremonial honor used for national holidays and dignitaries.

Historically, 21 appears in various calendars and literary references as a symbol of maturity and completion, and it is used to denote years such as 21 CE in historical timelines.