The CCITT, considering that standardization of the signalling systems to be used for international automatic and semiautomatic telephone working is necessary to keep to a minimum the number of different types of equipment serving the various routes at any one exchange; that the following signalling systems have been standardized and are applicable for general use in international automatic and semi-automatic working: Signalling System No. 4, standardized by the CCIF in 1954; Signalling System No. 5, standardized by the CCITT in 1964; Signalling System No. 6, standardized by the CCITT in 1968; Signalling System No. 7, standardized by the CCITT in 1980.