(Notas de euro)   ("Note" is the European, i.e., British English word; "bill" is American English.)
  The euro (€) notes have a letter (more recently, two letters) in the beginning of their identification. The letter is equivalent to a figure, so that, summing all the figures, one gets a multiple of 9 for (elementary !) security.

  As there are only 24 letters available (O and Q having been excluded for similarity with zero —maybe also I), this intelligent system will have to be revised or abandoned upon the adhesion of more countries.

Reference: BUESCU, Jorge, 2003, «Da Falsificação de Euros aos Pequenos Mundos», Gradiva, Lisboa.
Casting out nines (Weisstein)
(Wikipedia) Casting out nines. Prova dos noves (Br.).   (Google) Casting out elevens

http://web.tecnico.ulisboa.pt/~mcasquilho/text/utilities/euro.php Created: 2003 — Last modified: 2018-01-17