This is the most fundamental check. The Luhn algorithm (also known as the "mod 10" algorithm) is a simple checksum formula used to validate a variety of identification numbers. It detects accidental input errors, such as a single mistyped digit or adjacent transpositions.
Developers use card checkers to test the front-end validation of payment forms. This ensures that users are alerted to typos immediately, improving the overall user experience.
The first six to eight digits of a card are known as the Bank Identification Number (BIN) or Issuer Identification Number (IIN). Chkr.cc matches this against a database of over 300,000 records to identify the issuing bank, card type (credit vs. debit), and country of origin.