veintiuno

Veintiuno is the cardinal number 21 in Spanish. It is formed from the combination of veinti- (a contraction of veinte) and uno. In usage, the form changes with gender: veintiún is used before masculine singular nouns (for example, veintiún libros), and veintiuna before feminine singular nouns (veintiuna historias). When the numeral stands alone, veintiuno is commonly written.

In mathematics and numeration, veintiuno denotes the integer 21. It is a composite number with divisors 1, 3, 7, and 21. It is a triangular number (the sixth triangular number) and a Fibonacci number (it appears in the Fibonacci sequence). In Roman numerals it is XXI, and in standard positional numeral systems it is represented as 21 in base 10 and 10101 in binary.

Veintiuno is also the name of a popular card game in many Spanish-speaking countries, closely related to Blackjack. The objective is to accumulate card values up to 21 without exceeding it, with face cards counting as 10 and aces counting as 1 or 11. Players typically aim to beat the dealer by reaching 21 or by having a total closer to 21 than the dealer, under house rules that may vary by country or venue.

Culturally, veintiuno appears in dates and numerical expressions in Spanish. The number 21 is associated with adulthood and legal privileges differently across countries, with some jurisdictions allowing certain rights at 18 and others at 21. The term also serves as a ordinary numeral in calendars, measurements, and sequence listings.