Plessey Barcodes
The name is also used to refer to a barcode symbology developed by Plessey, which is still used in some libraries and for shelf tags in retail stores, in part as a solution to their internal requirement for stock control. The system was first used in the early 1970s by J.Sainsbury to identify all of its products on supermarket shelves for its product restocking system. The chief advantages are the relative ease of printing using the dot-matrix printers popular at the time of the code's introduction, and its somewhat higher density than the more common 2 of 5 and 3 of 9 codes.
Plessey barcodes use two bar widths. Whitespace between bars is not significant. The start element is a wide bar, and the stop element is two narrow bars. In between, the bars are in groups of four. High order bars appear leftmost. Narrow bars are zero and wide bars are 1.
This symbology is not self checking, though a modulo 10 or modulo 11 checksum (or some combination of both checksums, depending on application) is usually appended.
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