In modern times, emphasis on the institution of marriage, generally described as a male-female bond, has obscured pair bonds formed by same-sex and transsexual couples, and that many heterosexual couples also bond for life without offspring, or that often pairs that do have offspring separate.
These six digits are subject to an international standard, ISO/IEC 7812, and can be used to determine the type of card from the number.
Unfortunately the actual ISO/IEC 7812 database is not publicly available, however there are unofficial lists, both commercial and free, including on Wikipedia.
Also before bothering to pay for clearing a charge run a Mod 10 checksum on the card number to make sure it could be legitimate.
Because it’s typically 24 characters, otherwise users who enter spaces will run out of room.However, one particularity of the human species is that pair bonds are often formed without necessarily having the intention of reproduction.