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.
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