UPC 2-Digit Supplement (Extension)

The UPC 2-digit supplement is an add-on code used exclusively on periodicals to identify the issue number, keeping the main barcode constant across issues.

2-digit supplemental barcodes should only be used with magazines, newspapers, and other periodicals. The 2-digit supplement represents the issue number of the magazine. This keeps the product code in the main barcode constant, so each issue of the magazine doesn't need its own unique barcode. The 2-digit supplement can still be used to track which issue is being sold, for example for sales analysis or restocking purposes.

In practice, this is sometimes an "internal" issue number rather than the printed issue number on the cover. In some cases the encoded issue number is simply incremented with each issue; in others it may represent the month of the year or the week number, depending on how frequently the periodical is published.

1. Left-hand guard bars, encoded as 1011.
2. First data character, encoded using the appropriate parity pattern (see below).
3. Character separator of 01.
4. Additional characters, encoded using the appropriate parity pattern, with separators of 01.

There is no specific "end" or "stop" character. The barcode is assumed to end when a character separator (01) is not followed by a valid character.

The component characters of the barcode are encoded using the "left-hand odd" and "left-hand even" encoding character sets from the EAN-13 encoding standard. Whether a character is encoded using odd or even parity depends on whether the barcode is a 2-digit or 5-digit supplement. Essentially, the parity of the characters forms a simple checksum used to validate the integrity of the barcode.