Parallel long/short/missing lines, such as the UPC (Universal Product Code) grocery product labels, that encode product ids or other information. They can be printed with ordinary ink, with printers or offset presses. They can be read with various laser or LED (Light-Emitting Diode) scanners. Usually you read bar codes with a bar-code reader that attaches to your computer via a USB (Universal Serial Bus), serial port, or sometimes the keyboard port.
UPC bar codes work like this:
|UPC 01234567890 with check digit 5|
|How UPC Codes Work|
. represents a thin white bar, | represents a thin black bar and .... a four-times wide white bar and |||| a four-times wide black bar. Note that each digit takes up the same amount of space, 7 slots worth. Each digit always ends with a black bar. There are 27, = 128 possible bar codes. UPC uses only 10 of them, selecting patterns that have multiple differences between them and ones with a mixture of thin and thick bars. However, to me 0 and 1 look far too much alike. The bar code have a lead in |.| and final |.| with 12 digits including a trailing check digit.
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