Personal identification number (Denmark)
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The Danish Personal Identification number (Danish: CPR-nummer or personnummer) is a national identification number, which is part of the personal information stored in the Civil Registration System (Danish: Det Centrale Personregister).
The register was established in 1968 by combining information from all municipal civil registers of Denmark into one..
It is a ten-digit number with the format DDMMYY-SSSS, where DDMMYY is the date of birth and SSSS is a sequence number. The first digit of the sequence number encodes the century of birth (so that centenarians are distinguished from infants), and the last digit of the sequence number is odd for males and even for females.
Any person registered as of 2 April 1968 (1 May 1972 in Greenland) or later in a Danish civil register, receives a personal identification number. Any person who is a member of ATP or is required to pay tax in Denmark according to the Tax-control Law of Denmark, but is not registered in a civil register, also receives a personal identification number.
The civil register lists only persons who:
- Are born in Denmark of a mother already registered in the civil register, or
- Have their birth or baptism registered in a ’Dansk Elektronisk Kirkebog (DNK)’ (Danish electronic church-book), or
- Reside legally in Denmark for 3 months or more (non-Nordic citizens must also have a residence permit)
Danish citizens, including newborn babies, who are entitled to Danish citizenship, but are living abroad, do not receive a personal ID number, unless they move to Denmark.
New development in 2007 
The sequence numbers used to be chosen (and still are, preferentially) so that the last digit of the sequence number functions as a check digit for the entire personal identification number. In this case, the number satisfies the equation 4x1 + 3x2 + 2x3 + 7x4 + 6x5 + 5x6 + 4x7 + 3x8 + 2x9 + x10 ≡ 0 (mod 11) where the xi are the ten digits of the complete ID number, and the coefficients (4, 3, 2, 7, …) are all nonzero in the finite field of order 11.
However in 2007 the available sequence numbers under this system for males born on 1 January 1965 ran out, and since October 2007 personal identification numbers do not always validate using the check digit. This had been predicted and announced several years in advance. Thus, most IT systems are presumed updated to accept numbers that fail the check-digit validation.
1 January was the first birth date to run out of sequence numbers because immigrants who do not know their exact date of birth are administratively registered with the fictitious birth date of 1 January. This made the date unusually frequent in the register.
Personal ID Number Certificate 
Personnummerbevis is the Danish term for the personal identification number certificate. Today this certificate is of little use in Danish society, as it has been largely replaced by the much more versatile Sygesikringsbevis (now named : "Sundhedskort" / "Health Card")", which contains the same information and more. Both certificates retrieve their information from the Civil Registration System. However, personnummerbevis is still issued today and has been since September 1968.
It is received upon registration with the Civil Registration System, either by birth or by moving to the country. It may only be issued once and change of address does not entail issuing a new one. One can however request a new one from the Ministry of Welfare or in some cases the municipality one lives in.
Personal Identification Number in Danish Society 
The number is an integral part of Danish society, and it is virtually impossible to receive any form of government service without one. Even in the private sector one would be hard pressed to receive services without such a number, unless it is minor daily business.
See also 
- Danish Act on the Civil Registration System, Det Centrale Personregister
- Validate CPR-numbers (Danish)
- The Civil Registration System in Denmark: 2. HISTORICAL BACKGROUND
- The Civil Registration System in Denmark: 4.2 The personal identification number
- The Civil Registration System in Denmark: 3. THE CIVIL REGISTRATION SYSTEM - PURPOSE
- Første personnummer uden modulus 11 kontrol er nu tildelt
- CPR, Danmarks folkeregister (Word document)