999 (emergency telephone number)
999 is Ireland's and the United Kingdom's emergency telephone number along with the EU standard 112. Either number can be used but 999 is more popular and better-known than 112.
It is also the emergency telephone number in some Commonwealth countries. It is an all-service number, meaning that it should be called in any situations where state-run emergency services are needed. The three main and best-known services are fire brigade, police and ambulance. Other available services include coastguards, mountain rescue and cave rescue (where locally relevant). Some situations such as a major car accident or a terrorist attack (including nuclear, biological, or chemical attacks) will require multiple and/or specialist services but the first point of call for reporting such incidents from the general public is still the 999 system. The number is operated by British Telecom and Cable and Wireless for the Home Office.
In some situations there will be specific instructions on nearby signs to notify some other authority of an emergency before calling 999. For example there are notices on bridges carrying railways over roads telling people that, if they see a road vehicle striking the bridge, they should call the railway authority (on a given number) first and then call 999 to inform the police.
It is also used in the Ghana, Hong Kong, Macau, Malaysia and Singapore.
History
999 was originally chosen as the emergency number during the height of the Blitz on London during the second World War. It was chosen because of the need to dial it in total darkness (because of the London blackout) and using the rotary dial type telephones of the period. In the dark, it was easiest to locate the numbers at either end of the dial rather than count the finger holes to get to some number in the middle of the range one through zero. Picking a number with three identical digits made it harder to mis-dial once your finger was in the correct hole in the dial. Numbers such as 1-1-1 and 2-2-2 that generate few dial pulses were considered undesirable because they could easily be generated by accident by (for example) jiggling the receiver rest. The number 0-0-0 (which would generate three sequences of ten pulses each) would have been ideal but could not be used because dialling the first Zero would call the operator before you'd get the chance to dial the next two digits.
9-9-9 was therefore the next most easy to dial number and hence the most logical candidate given the technology of the day.
In a modern situation where dial phones and pulse signalling phones are rarely seen and most telephones are in well lit areas, almost any short number could be chosen though the possibility of creating dialing pulses by accident is still a possibility (phone exchanges still accept pulse dialing).