Series A - Organization of the work of the ITU-T
Series B - Means of expression
Series C - General telecommunication statistics
Series D - General tariff principles
Series E - Telephone network and ISDN
Series F - Non-telephone telecommunication services
Series G - Transmission systems and media
Series H - Transmission of non-telephone signals
Series I - Integrated services digital network
Series J - Transmission of sound-programme and television signals
Series K - Protection against interference
Series L - Construction, installation and protection of cables and other elements of outside plant
Series M - Maintenance: international transmission systems, telephone circuits, telegraphy, facsimile and lease circuits
Series N - Maintenance: international sound-programme and television transmission circuits
Series O - Specifications of measuring equipment
Series P - Telephone transmission quality
Series Q - Switching and signalling
Series R - Telegraph transmission
Series S - Telegraph services terminal equipment
Series T - Terminal equipments and protocols for telematic services
Series U - Telegraph switching
Series V - Data communication over the telephone network
Series X - Data networks and open system communication
Series Z - Programming languages