In performing the Mod 10 test, which digits of the PAN are doubled?

The Mod 10 test, also known as the Luhn algorithm, is a simple checksum formula used to validate a variety of identification numbers, including credit card numbers. In this test, the digits of the Primary Account Number (PAN) are processed to help identify any errors in the number.

For the Mod 10 calculation, every second digit from the right is doubled, starting with the second digit. This is crucial in ensuring that the checksum appropriately indicates the validity of the PAN. The doubling of these alternate digits can lead to numbers larger than 9, and in such cases, the digits are summed so that only a single digit contributes to the final total.

Thus, when performing the test, the correct approach involves focusing on this specific sequence, which clarifies why the digits that are doubled begin with the second one moving towards the left, rather than including every digit or starting from the first. Recognizing this method is essential for accurately implementing the Luhn algorithm in practical scenarios, such as credit card validation.