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Summary
Promoted on the Internet and elsewhere is the erroneous claim that every barcode in the Universal Product Code (UPC-A) has the number 666 embedded in it. A look at how the numbers in the UPC barcode are constructed will show why this claim is wrong.
Someone glancing casually and a bit carelessly at a UPC barcode containing the number six will note that there are two thin vertical bars almost directly above the number six. Here, for example, is the barcode from a box of After Eight dinner mints. Note the two thin bars above the six in the 48600:
The casual glancer may therefore assume that the number six is represented by two thin vertical bars. Noticing that there are two thin vertical bars at each end of the barcode might then convince the glancer that these must be sixes also. The glancer then may finally note that in the confusing middle of the barcode there are also (at least) two thin vertical bars. Hence the hasty conclusion: the barcode pattern contains a six on each end and a six in the middle.(In this particular example, the little check digit six on the far right may also be confusing. But read on for clarification.)
Note that on the right there might appear to be two thin bars just above the six, but on the left, the pattern above the six looks like a thin bar and a very wide bar. What is actually there, however, are the four elements of the number six, represented on the right side of the barcode by a thin line, a thin space, a thin line, and a very wide space, and on the left side by a thin space, a thin bar, a thin space, and a very wide bar.
To make this even plainer, here is a barcode containing all sixes:
Note that the sixes on the right are all made up of a thin line, a thin space, a thin line, and a very wide space, and those on the left are made up of a thin space, a thin bar, a thin space, and a very wide bar.
The lines at each end of the barcode are guard bars, consisting of a thin line, a thin space, and a thin line. Since there are only three elements instead of four, no number of any kind is represented. (No very wide space or very wide bar is there to complete the number six, for example.)
In between the first five numbers (which represent the manufacturer's identifying number--who made the product) and the last five numbers (which represent the specific product code--such as a blue, medium ball-point pen) is another set of guard bars, consisting of a thin space, a thin line, a thin space, a thin line, and a thin space. Here, even though there are five elements (two bars and three spaces), they do not represent a number because none of the numbers are made up of all thin elements (see below).
Since the lines and spaces appear in four different widths, we could use the words thin, medium, wide, and very wide to represent them. We can also use numbers, though, with 1 representing thin, 2 representing medium, 3 representing wide, and 4 representing very wide. That plan allows us to represent each number this way:
Let's return to the padlock barcode, enlarged and detailed here. Note that the barcode contains two sixes, one on each side of the middle guard bars. As mentioned above, you can see the difference in representation of the same 1-1-1-4 code for a left-hand six, where the very wide segment is a bar instead of a space, and the right hand six, where the very wide segment is a space rather than a bar. Yet you are looking at the same number.
Note also that every bar and every space in the barcode is accounted for, and that there are no extra sixes anywhere to be found. (The first number, on the left next to the barcode, is the barcode type, such as a regular item, a coupon, etc., and the last number, on the right after the barcode, is a check digit, calculated from the barcode numbers and helping to insure accuracy of scanning. In both cases the numbers themselves are encoded inside the guard bars, making the barcode twelve digits overall.)