The supplemental bar code may either encoded 2 or 5 digits of information.

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.

The first digit of the supplement indicates the currency in which the price is expressed. A "0" represents a price expressed in British Pounds whereas a "5" represents a price expressed in U.S. dollars. The remaining 4 digits of the supplement indicate the price. For example "51195" indicates a suggested retail price of US$11.95 (US Dollars). I guess this means the people at UCC think no book is worth more than $99.99, and they're probably right-but they should let some of the publishers know that!

A supplementary code of "90000" means the book has no suggested retail price. In most stores, this does not mean you make take the book without paying .

A supplementary code of "99991" indicates a complimentary copy of the book. This normally means "free." A retailer that is charging you for a book marked "99991" merits suspicion, but we still don't advocate walking off with such a book without paying for it.

Supplementary codes of 90001 to 98999 are used by some publishers for internal purposes. As an external user it is not generally possible to get much useful information from such supplemental bar codes due to the very fact that they are encoding some information internal to the publisher.

The supplementary code "99990" is used by the National Association of College Stores to mark used books. Don't pay "new" prices for books marked with this supplementary code.

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.

The component characters of the bar code 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 bar code 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 bar code.

REMAINDER	CHARACTER 1 PARITY	CHARACTER 2 PARITY
0	Odd	Odd
1	Odd	Even
2	Even	Odd
3	Even	Even

Thus, encoding the value "34" as a 2-digit UPC Supplemental bar code would be as follows:

1. Left-guard bars, encoded as 1011.
2. 1st Digit [3] encoded as even parity, 0100001.
3. Inter-character separator, encoded as 01.
4. 2nd Digit [4] encoded as odd parity, 0100011.

First, a checksum value must be calculated for the bar code.

1. Consider the right-most digit of the message to be in an "odd" position, and assign odd/even to each character moving from right to left.
2. Sum the digits in all odd positions, and multiply the result by 3.
3. Sum the digits in all even positions, and multiply the result by 9.
4. Sum the totals calculated in steps 2 and 3.
5. The check digit is the digit in the units position of the value calculated in step 5 (i.e., if the result in step 4 was 37, the check digit is 7).

CHECK DIGIT	PARITY PATTERN FOR EACH CHARACTER
	1	2	3	4	5
0	Even	Even	Odd	Odd	Odd
1	Even	Odd	Even	Odd	Odd
2	Even	Odd	Odd	Even	Odd
3	Even	Odd	Odd	Odd	Even
4	Odd	Even	Even	Odd	Odd
5	Odd	Odd	Even	Even	Odd
6	Odd	Odd	Odd	Even	Even
7	Odd	Even	Odd	Even	Odd
8	Odd	Even	Odd	Odd	Even
9	Odd	Odd	Even	Odd	Even

Barcode	5	1	2	3	4
Position	O	E	O	E	O
Weighting	3	9	3	9	3
Calculation	5 * 3	1 * 9	2 * 3	3 * 9	4 * 3
Weighted Sum	15	9	6	27	12

1. Left-guard bars, encoded as 1011.
2. 1st Digit [5] encoded as odd parity, 0110001.
3. Inter-character separator, encoded as 01.
4. 2nd Digit [1] encoded as odd parity, 0011001.
5. Inter-character separator, encoded as 01.
6. 3rd Digit [2] encoded as even parity, 0011011.
7. Inter-character separator, encoded as 01.
8. 4th Digit [3] encoded as odd parity, 0111101.
9. Inter-character separator, encoded as 01.
10. 5th Digit [4] encoded as even parity, 0011101.