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Barcode Education » UPC 2-Digit Supplement (Extension)
Introduction
2-digit supplemental barcodes should only be used with magazines, newspapers and other such periodicals. The 2-digit supplement represent the issue number of the magazine. This is useful so that the product code itself (contained in the main barcode) is constant for the magazine such that each issue of the magazine doesn't have to have its own unique barcode. Nevertheless, the 2-digit supplement can be used to track which issue of the magazine is being sold, perhaps for sales analysis or restocking purposes.

In reality, this is sometimes an "internal" issue number. The 2-digit extension is not always the same as the "Issue Number" that is printed somewhere on the cover. Sometimes the encoded issue number just gets incremented with each issue. In other cases, the encoded issue number may just be the number of the month of the year, or the week number, depending on the frequency with which the periodical is published.

Encoding

1. Left-hand guard bars, encoded as 1011.
2. First data character, encoded using the appropriate parity pattern (see below).
3. Character seperator of 01.
4. Additional characters, encoding 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) doesn't follow 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 varies depending on whether the barcode is a 2-digit or 5-digit supplement. Essentially, the parity of the characters constitutes a rather crude checksum which is used to validate the integrity of the barcode.

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