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Plessey is also a name for a barcode symbology developed in Britan. It is still used in some libraries and for shelf tags in retail stores. Its 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, and are interpreted in BCD. 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 (depending on application) is usually appended.

Plessey is also a former British computer manufacturer.

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