What Credit Card Numbers Mean
Although phone companies, gas companies and department stores have their own numbering systems, ANSI Standard X4.13-1983 is the system used by most national credit-card systems.
Here are what some of the numbers stand for:
The first digit in your credit-card number signifies the system:
3 - travel/entertainment cards (such as American Express and Diners Club)
4 - Visa
5 - MasterCard
6 - Discover Card