13

Thirteen is the natural number that follows twelve and precedes fourteen. In mathematics, 13 is a prime number. It is the sixth prime, and the thirteenth in the sequence of primes. It appears in the Fibonacci sequence as F7 = 13. It is also a Mersenne prime exponent, since 2^13 − 1 = 8191 is prime. In base 10 it is written as 13; in Roman numerals XIII; in hexadecimal D.

In science, the element with atomic number 13 is aluminum (Al). Aluminum is the most abundant metal in the Earth's crust and is valued for its light weight and corrosion resistance, making it widely used in transportation, packaging, and construction.

In culture and superstition, 13 is associated with bad luck in many Western cultures, and Friday the 13th has become a prominent cultural reference. Many buildings omit the 13th floor, and the number appears in the context of historical milestones such as the original thirteen American colonies and the thirteen stripes on the early United States flag. In some cultures, 13 is considered lucky or significant in other ways.

In geometry and astronomy, there are 13 Archimedean solids. Messier 13, the Great Hercules Cluster, is a well-known globular star cluster in the constellation Hercules.

In computing, the ASCII code 13 represents the carriage return, used to move the cursor to the start of a line in text processing.