999

999 is the natural number that comes after 998 and before 1000. In decimal notation it is a repdigit, consisting of three identical digits. It can be expressed as 10^3 − 1, and its prime factorization is 3^3 × 37. The sum of its digits is 27, so its digital root is 9. It is divisible by 9, 3, 27, 37, 111, and 333.

In Roman numerals, 999 is CMXCIX.

999 has various uses beyond a simple numeral. It is the national emergency telephone number in the United Kingdom and Ireland, used to contact police, fire, ambulance and coastguard services. It is one of several emergency numbers used in international practice; calls are routed to local emergency services, and operators can provide assistance or dispatch responders. A separate universal number, 112, also works in many countries.

Cultural references include its use as the name of a British punk rock band formed in 1977, known simply as the 999. The band released several singles in the late 1970s, contributing to the UK’s late-1970s punk scene.

As a mathematical object, 999 is also notable for being one less than 1000, a common threshold in computing and counting systems, and for its role as a three-digit repdigit in decimal notation.