991

991 is a natural number that comes after 990 and before 992. In mathematics, 991 is a prime number, meaning it has no divisors besides 1 and itself. In various numeral systems, 991 can be represented as 1111011111 in binary and 0x3DF in hexadecimal; in Roman numerals, it is CMXCI. As a year, 991 CE falls in the late 10th century of the Anno Domini era, a period associated with the Middle Ages in Europe; events would vary by region.

In popular culture and industry, 991 is most commonly associated with the Porsche 991, the internal designation for the seventh generation of the Porsche 911 sports car. The 991 generation was introduced in 2011 and produced through 2019, with a mid-cycle refresh known as 991.2 debuting around 2016. The 991 range introduced broader use of lightweight materials and updated styling, and it was succeeded by the Porsche 992 in 2019.

The number 991 also appears in other fields as a model or designation, including catalog numbers, part numbers, or codes, but these usages vary widely by manufacturer and context.