The secret information that the letter of your ID reveals

The National Identity Document (DNI) is a public, personal and non-transferable document, issued by the Ministry of the Interior (@interiorgob), which certifies the identity, personal data and Spanish nationality of its owner. Of all the data and letters it contains, those that we always need to remember when we do different procedures (activate bank accounts, make electronic purchases, make arrangements with any of the public administrations, etc.) is the eight-digit number followed by a letter. But, What exactly does that letter mean?

Rotated obverse ID


That alphabetic character has given rise to all kinds of interpretations. If you are from Madrid (or Bilbao), for example, and the letter that appears on your identity card is M (or B), perhaps you thought that it referred to the town in which you were registered, for example. At the end of the day, this card with the same dimensions as our credit cards that we carry has its antecedent in a document with which it was intended to have census information of citizens, back in the decade of the 40s of the last century. In fact, among the first people who were obliged to obtain a DNI were those who, due to their profession, habitually changed their address. However, that letter that accompanies the eight numbers of our identity document is a check digit that verifies our tax identification number, but The curious thing here is the way in which that mysterious letter is assigned to each of us.

To determine the alphabetical character of the control that accompanies our personal number is used, according to the Ministry of the Interior, the following calculation: Our 8-digit number is divided by 23 and the remainder resulting from the division is replaced by a letter that is determined by inspection using the following table:

The secret information that the letter of your ID reveals

Thus, following the example of the Ministry: «if the DNI number is 12345678, divided by 23 gives the remainder 14, then the letter would be Z: 12345678Z.

Why can’t anyone have a Ñ on their ID

If you have observed the previous table, you will have seen that four letters of the alphabet do not appear in it: I, Ñ, O and U. In the case of “I” and “0”, their use was discarded for something obvious: they can be confused with the one (1) and the zero (0) and read as one more number. The “Ñ” thing is a classic: the problem is not only that it could be confused with the letter “N”, but it could also cause problems when using computer systems from other countries where that letter does not exist. In the case of U, the reason would be to remove a fourth letter simply so that 23 would remain to be distributed, which, being a prime number, is more suitable for doing the calculations.