By the early twentieth century, the complexity of international telephone service and long-distance telegraphy made it necessary to carry out international studies in the interval between Union conferences in order to develop relevant international standards. Two consultative committees were therefore created by the 1925 International Telegraph Conference in Paris:
- International Telephone Consultative Committee (CCIF) - to study and develop standards for terminal equipment, transmission quality and tariffs for long-distance telephony
- International Telegraph Consultative Committee (CCIT) – to deal with the technical and operational aspects of telegraphy, to standardize phototelegraphy and to establish tariffs and international terminology.
These Committees each established a structure with Study Groups carrying out research and developing proposed standards (called Recommendations) and regular Plenary Assemblies which approved the standards and organized the work of the Study Groups.
In view of the basic similarity of many of the technical problems faced by the CCIF and CCIT, a decision was taken in 1956 to merge the two committees into a single committee:
- International Telegraph and Telephone Consultative Committee (CCITT) - Its duties were to study technical, operating and tariff questions related to telegraphy and telephone and to issue recommendations on them.
In 1992, the Union's Additional Plenipotentiary Conference in Geneva undertook a reform of ITU to give the Union greater flexibility to adapt to an increasingly complex, interactive and competitive telecommunications environment. ITU's three main areas of activity were organized in "Sectors": telecommunication development, radiocommunication, and telecommunication standardization. The CCITT was renamed the Telecommunication Standardization Sector (ITU-T).
- Telecommunication Standardization Sector (ITU-T) - Its functions are to study technical, operating and tariff questions and adopt recommendations on them with a view to standardizing telecommunications on a worldwide basis.
The Telecommunication Standardization Bureau (TSB) is the secretariat for the Sector.