Volume 0, X Protocol Reference Manual describes the X Network Protocol, which underlies all software for Version 11 of the X Window System. This fourth edition is updated for R6 and can be used with any release of X. Contents are divided into three parts: Part One provides a conceptual introduction to the X Protocol. It describes the role of the server and client and demonstrates the network transactions that take place during a minimal client session. Part Two contains an extensive set of reference pages for each protocol request and event. It is a reformatted and reorganized version of the X Consortium's Protocol specification. All material from the original document is present in this manual, and the material in the reference pages is reorganized to provide easier access. Each protocol request or event is treated as a separate, alphabetized reference page. Reference pages include the encoding of requests and replies. Part Three consists of several appendices describing particular parts of the X Protocol, along with several reference aids.Note: This fourth edition does not contain the Inter-Client Communication Conventions Manual (ICCCM) or the X Logical Font Description Convention (XLFD). This material will be included in an upcoming O'Reilly book.