The number uses ten digits, YYMMDD-NNGC. The first six approximate the birth date in YYMMDD format; for individuals with an unknown birth date or those born on days where there aren't any unique numbers left a date close to the birth date may be used. Digits seven to nine (NNG) are used to make the number unique, where digit nine (G) is odd for men and even for women. For numbers issued before 1990, the seventh and eighth digit identify the Swedish county of birth, with different digits for foreign-born people, but privacy-related criticism caused this system to be abandoned for new numbers. The tenth digit (C) is created using the Luhn, or "mod 10", checksum algorithm.