To encode an EAN-13 barcode, the digits are first split into 3 groups; the first digit, the first group of 6 and the last group of 6. The first group of six is encoded using a scheme whereby each digit has two possible encodings, one of which has even parity and one of which has odd parity. The first digit is encoded by selecting a pattern of choices between these two encodings for the next six digits, according to the table below. (Unlike the other digits, the first digit is not represented directly by a pattern of bars.) All digits in the last group of six digits are encoded using a single set of patterns which are the same patterns used for UPC.
If the first digit is zero, all digits in the first group of six are encoded using the patterns used for UPC, therefore, a UPC barcode is also an EAN-13 barcode with the first digit set to zero.