National identification numbers or INSEE numbers.
Each French person receives at birth a national identification number, called "Social Security number", which comes from his registration to the NIR (National Repertory). The number was created by René Camille in 1941 under the Vichy Regime. This INSEE number is composed of 13 digits + a two-digit key. Although the total number is of 15 digits, the rationale behind it makes it easy for individuals to remember at least the first seven digits of it (they just have to know their sex, year and month of birth, and department of birth). Since this number is used in many administrative procedures (whether by the state or by private enterprises), most people know by memory part of this identification number.
Their format is as follows: syymmlloookkk mm, where
o s is 1 for a male, 2 for a female,
o yy are the last two digits of the year of birth,
o mm is the month of birth, usually 01 to 12 (but there are special values for persons whose exact date of birth is not known),
o ll is the number of the départment of origin : 2 digits, or 1 digit and 1 letter in metropolitan France, 3 digits for overseas.
o ooo is the COG number (see below) of the commune of origin (a department is composed of various communes) : 3 digits in metropolitan France or 2 digits for overseas.
o kkk is an order number to distinguish people being born at the same place in the same year and month. This number is the one given by the Acte de naissance, an official paper which officialize a birth (and is needed through-out life for various administrative procedures, such as getting an identity card).
o 'mm' is the "control key", 01 to 97, equal to 97-(the rest of the number modulo 97) or to 97 if the number is a multiple of 97.
There are exceptions for people in particular situations.
The "sex" codes (s: 1 for male, 2 for female) can be given in special occasions for temporary registrations, such as for someone who a person who works as a wage-earner but is not registered for miscellaneous reasons. Under Vichy France, but only in Algeria (not in metropolitan France) this s code was also used to register Jews, Algerian Muslims, foreigners, or ill-defined people. Thus, 3 or 4 was given to non-Jews indigenous people of Algeria (improperly called "Muslims") and of all colonies; 5 or 6 for indigenous Jews; 7 or 8 for foreigners; 9 or 0 for miscellaneous and ill-defined status (people entering into none of these class).
They are also specific codes for people whose date or place of birth is unknown, although this is today more and more rare (for example, birth code is superior to 20 if month of birth is unknown, and communal code is 990 if the commune of origin is unknown). For overseas departments, the department number has got three digits, and the communal number two digits (since 1950). People born abroad have a departmental code of 99, and the communal code is replaced by the code of the country of birth which has three digits. Before 1964, departmental codes from 91 to 96 were used for Algeria, Tunisia and Morocco.
If in a specified month the number of total births is superior to 999, an extension common code is created.
The last code is obtained by a mathematical method (dividing by 97 the number formed by the first 13 digits, take the left-over of this division, and then the "complement at 97", that is the difference between 97 and the left-over of the division): this gives the control key code.