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Credit card numbers follow certain patterns. A credit card number must have between 13 and 16 digits. It must start with: 4 for Visa cards 5 for Master cards 37 for American Express cards 6 for Discover cards In 1954, Hans Luhn of IBM proposed an algorithm for validating credit card numbers. The algorithm is useful to determine if a card number is entered correctly or if a credit card is scanned correctly by a scanner. Almost all credit card numbers are generated following this validity check, commonly known as the Luhn check or the Mod 10 check which can be described as follows (for illustration, consider the card number 4388576018402626): 1. Double every second digit from right to left. If doubling of a digit results in a two-digit number, add up the two digits to get a single-digit number 2 2 4 4 2 8 6 2 12 (1 +2 3) 5 2 10 (1 +0 1) 8 2 16 (1+6-7) 4 2 8 2. Now add all single-digit numbers from Step 1 4 +4+8+2+31+7+ 8 37 3. Add all digits in the odd places from right to left in the card number