Portuguese ID Card
The most important identification document of a Portuguese citizen is the identity card BI ("Bilhete de Identidade").
Each BI card has an identification number (on the back, top left) which remains the same throughout the individual's life (even if the BI is renewed because of other reasons, for example, change of marital status or residence).
Given the importance of BI, it is very important that the use and transmission of this number occur without errors. This is the main purpose of the check digit* appended to the identification number (in a small square). Note that with the kind of big numbers as in BI cards, the possibility of someone to transcribe them with errors is real.