Nineteen

Nineteen is the natural number that follows eighteen and precedes twenty. In decimal notation it is written as 19. It is a prime number and appears in many mathematical and everyday contexts.

Etymology and numerals: The word nineteen stems from the combination of nine and the suffix -teen, from Old English, indicating ten. In other numeral systems, it is written as XIX in Roman numerals, 10011 in binary, and 0x13 in hexadecimal. The chemical element potassium has atomic number 19.

Mathematics: 19 is the eighth prime number and forms a twin prime pair with 17. It is a centered hexagonal number (1, 7, 19, 37, …). As a prime congruent to 3 modulo 4, it cannot be expressed as a sum of two squares.

Culture and history: The 19th Amendment to the U.S. Constitution, ratified in 1920, granted women the right to vote. The Metonic cycle—the 19-year period after which lunar phases recur on the same calendar dates—has been influential in astronomy and calendar calculations.

In science and culture: Potassium (chemical symbol K) has atomic number 19. Messier object M19 is a globular cluster in Ophiuchus. The 19th letter of the Latin alphabet is S.