Issue numbers are used by some European credit card companies but not by MasterCard. If an online checkout asking for an issue number for a MasterCard is encountered, it can be left blank, or the numeral 0 can be used.
Some European online retailers require the entry of an issue number. Most credit cards, including MasterCard, do not use issue numbers. Depending on the retailer, either leaving the field blank or entering the numeral 0 allows the transaction to be completed. Issue numbers are more commonly used with debit cards in the United Kingdom and Europe. Americans are generally not familiar with the term issue number as it relates to credit card transactions.