Twenty

Twenty is the number that follows nineteen and precedes twenty-one. In base-10 notation it is written as 20 and is a two-digit positive integer. In Roman numerals it is XX. The word twenty is common in everyday counting and in expressions such as “score,” which historically denotes twenty.

Etymology and related forms: The English word twenty comes from Old English twentig, literally meaning two tens. It is cognate with Dutch twintig and German zwanzig, reflecting a common Proto-Germanic heritage for the number two and ten.

Mathematical properties: Twenty is an even, composite number with prime factorization 2^2 × 5. It is not a prime number. In decimal representation it is a two-digit number; in other bases its digit representation and factors differ accordingly.

Cultural and linguistic notes: Twenty features in various counting systems and idioms. Some languages throughout history use vigesimal counting, based on twenty, rather than a strict base-10 system; for example, certain terms in Danish are derived from twenty. In French, the influence of twenty appears in compounds like quatre-vingts for eighty, illustrating how counting by twenties can shape numbers in a language.

Science and measurement: In chemistry, calcium has atomic number 20 and symbol Ca. In time, twenty minutes constitute one third of an hour. The number also often appears as a convenient round figure in statistics, demographics, and everyday planning.