The letter used by the ITU to categorize standards relating to data communications over digital circuits; the number following the letter identifies a particular standard. Some of the more important standards in the X-series are listed.

• X3 PAD control;

• X25 data signalling between the equipment associated with the PTT and the user;

• X28 communication between a PAD and an ASCII device;

• X29 communication between two PADs;

• X75 communication between networks using X25;

• X121 standards for addressing in an X25 network;

• X400 message handling services: all standards in the range X400 to X499 relate to various aspects of message handling;

• X500 directory services: all standards in the range X500 to X599 relate to various aspects of directory services.