CLABE
Mexico's standardised 18-digit bank account number, required to send money through the SPEI payment system.
The Clave Bancaria Estandarizada was introduced by Banco de México in the early 2000s, when the country's banks still used their own incompatible account formats. The structure is fixed: three digits for the bank, three for the plaza (the city where the account was opened), eleven for the account and a final check digit. That digit uses a weighted modulo 10, multiplying the digits by 3, 7 and 1 in turn.
Mexico does not use IBAN, so CLABE fills the same role domestically: a number that identifies the bank and can be validated before any money leaves. The plaza code is a leftover from an era when branches mattered more than they do now, but it stays because nobody wants to renumber every account in the country.