The Plessey Company in England originally developed Plessey Code with formal specifications first dated in 1971. It gave rise to several variations including the MSI, Anker, and Telxon codes. Of these, the MSI Plessey is till in use in the United States; it is used in libraries, and is often used for retail grocery shelf marking.

MSI Plessey Code is a pulse-width modulated non-self checking code. Each character is represented by 4 bars; a narrow bar represents a binary 0 and a wide bar represents a binary 1. The bars have the binary weights 8-4-2-1. It is possible to encode the digits 0 through 9 and the letters A through F, although this code is most often used just for numeric information.

In MSI code the zero bit is a one-unit bar followed by a two-unit space and the one bit is a two-unit bar followed by a one-unit space. Complete four bit characters are thus 12 units wide, which is large for a numeric symbology. The MSI symbol includes a start pattern, data characters, one or two check digits, and a stop pattern. A MSI Plessey barcode always includes a Modulo 10 checksum and may include a second checksum.

MSI-PLESSEY is a variable length, numeric only, symbology. The symbology is one of the earliest bar code symbologies ever developed and is based on a four bit binary number scheme. Each symbol is framed by a start and a stop pattern and contains a check character that is calculated from the values of each of the encoded data digits. MSI-Plessey is rarely used in anything other than grocery store shelf marking applications. In fact most modern bar code readers do not provide support for reading MSI-Plessey symbols.