An Acronym for ITU Telecommunication Standardization Sector, formerly known as CCITT. ITU is an Acronym for 
International Telecommunications Union.
The ITU-T is responsible for the X509 and H.323 standards in the computing world, as well as lots of things to do with POTS standards and interoperability.
ITU specifications are horrible to read, they refer all over the place to each other, and feel like they should be read by a computer. Thing is, many of them are designed to be parsed by an ASN.1 parser.
ITU-T standards currently in force are freely downloadable from here.
The ITU-T standards all start with a letter a dot and some numbers:
| Letter | Meaning | |
| A | Organization of the work of ITU-T | |
| B | Means of expression: definitions, symbols, classification | |
| C | General telecommunication statistics | |
| D | General tariff principles | |
| E | Overall network operation, telephone service, service operation and human factors | |
| F | Non-telephone telecommunication services | |
| G | Transmission systems and media, digital systems and networks | |
| H | Audiovisual and multimedia systems | |
| I | Integrated services digital network | |
| J | Cable networks and transmission of television, sound programme and other multimedia signals | |
| K | Protection against interference | |
| L | Construction, installation and protection of cables and other elements of outside plant | |
| M | TMN and network maintenance: international transmission systems, telephone circuits, telegraphy, facsimile and leased circuits | |
| N | Maintenance: international sound programme and television transmission circuits | |
| O | Specifications of measuring equipment | |
| P | Telephone transmission quality, telephone installations, local line networks | |
| Q | Switching and signalling | |
| R | Telegraph transmission | |
| S | Telegraph services terminal equipment | |
| T | Terminals for telematic services | |
| U | Telegraph switching | |
| V | Data communication over the telephone network | |
| X | Data networks and open system communications | |
| Y | Global information infrastructure and Internet protocol aspects | |
| Z | Languages and general software aspects for telecommunication systems |
Some well known standards:
