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The No. 1 Electronic Switching System (ESS) is the first electronic machine produced in quantity by the Bell System for central office switching. A 2-wire version is being used for Bell System commercial customers and a 4-wire version for a variety of government and military applications. The system is composed largely of solid-state devices and is controlled by a stored program. The nine No. 1 ESS's now in use have been giving good service to their commercial and government users. Judging from customer reaction and from various technical performance criteria, the No. 1 ESS has met its design objectives.