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Typically, passport numbers are not just running indices of the passports issued, but have certain rules regarding relating the numbers, use of letters, and/or suffixes and prefixes.

Is it the case, that you can determine which world state issued a passport based solely on the "number" (i.e. digit-and-letter combination referred to as the passport number)? Or can different states issue passports with the same number?

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The passport number itself is not normalised, can have different lengths and structures depending both on the country and when it was issued, can use the same number for different passports issued by different countries, and usually does not include any information about the issuing country.

However, the MRZ (machine readable zone, the two lines at the bottom of the bio page) is standardised and includes both the issuer country and the passport number (and more information).

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    +1 Whenever a form (paper or online) has asked for my passport number it has also needed the issuing country, unless that is already determined by context. – Patricia Shanahan Feb 22 at 9:27
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    But do different countries actually have overlapping valid passport numbers? As opposed to complete MRZs? – einpoklum Feb 22 at 9:30
  • @einpoklum it is difficult to know (unless you find actual collisions) as the structure of those numbers is usually not published, but you most definitely can’t assume they are globally unique without the country code. Note that many passport actually have a line in the bio page that contains P, the issuing country’s 3-letter ISO code, and the passport number. That combination should be unique. – jcaron Feb 22 at 9:43
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    According to docs.microsoft.com/en-us/microsoft-365/compliance/… there are many countries which use 8 or 9 digit numbers. This give 100 million or one billion numbers, which is most probably not enough for uniquely numbering all passports in the world. – jcaron Feb 22 at 9:48
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    Actually probably even less, given that in many cases one of the digits is a checksum or similar validity verification number. – jcaron Feb 22 at 9:57

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