No "counter" series used the consonants Q and R (and Q has never been allowed in any way, the apparent reason for this being its resemblance to the vowel O and the digit 0), while two-letter combinations ending in the vowels A, E, I and O were also forbidden, apparently to avoid the forming of potentially offensive Spanish words when combined with some province codes (such as codice_11, meaning "bad one", or codice_12, meaning "arse"). This meant that, for instance, codice_13 was followed by codice_14, while codice_15 was followed by codice_16 and then codice_17, and codice_18 was followed by codice_19.