System X circuit board from digital switching subsystem, probably made by Plessey Company Limited, Liverpool, England, 1980.

System X was British Telecom's first digital switching exchange for telephones. It was thought up by the Advisory Group on System Definitions, an alliance involving the Post Office and industry groups, set up in 1968. The system itself was designed in collaboration between GEC, Plessey and STC - the three major suppliers to the Post Office. The first System X exchange came into service on July 1 1980 at London's Baynard House. It was called the 'great white hope' for British manufacturers, with the intention of worldwide sales, but unfortunately development took too long, and manufacturers looked elsewhere. Nevertheless System X became standard in Britain and remained in widespread use well into the 21st century.