Card number length is set by the network, not debit or credit

Debit cards don't have 12 digits and credit cards don't have 15: the network decides the number

The question sounds like Banking 101, but the answer dismantles the premise. There is no standard that assigns a different number of digits to debit cards and credit cards. What determines how many digits the card carries is the network that issues it, not the way the money is settled. Visa and Mastercard, the two dominant networks in Spain, work with 16 digits for both debit and credit. The exception worth keeping in mind is American Express, which uses 15. And there are older issues or other systems that run 14 or even 19 digits.

The nuance matters more than it seems. Anyone who mistakes the length of the number for the card type will end up looking for patterns that don't exist.

What is Maestro and why it gets mistaken for a third network

The confusion has a name: Maestro. Many people place it as a third independent network alongside Visa and Mastercard. It isn't. Maestro is Mastercard's debit brand, which is why it appears less and less as a separate logo on cards. Hence the feeling that "Mastercard is credit and Maestro is debit," a simplification that falls apart as soon as you look at the contract: there are Mastercard debit and credit cards, and the brand grouping them is the same.

A product's commercial name doesn't always match the nature of the money it moves. A card bearing the Mastercard logo may be charging instantly against your current account or opening a thirty-day line of credit. What changes is not the number of digits: it's where the balance comes from.

Debit and credit: the difference lies in the balance, not the digits

With a debit card you spend money that is already in your account. With a credit card you spend regardless of the state of that account until settlement time comes around. That is the real axis of the distinction, not the count of digits on the front. As long as the available balance covers the amount, the behavior at the terminal is identical for the cardholder; the difference shows up later, on the statement.

The practical consequence is that the number of digits is neither an alarm signal nor a clue to anything. It serves to identify the network and the issuer: the first digits of the number identify the operator and the bank, and the first six form the BIN, the Bank Identification Number. The last digit is calculated from the others, a check digit that helps detect typing errors.

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There the matter rests, in the realm of technical identification. Nobody has yet found a rule that lets you guess the card type by counting digits, and anyone looking for one will keep counting in vain.