Inter-Client Communication Conventions Manual
In computing, the Inter-Client Communication Conventions Manual (almost universally referred to by its initials, ICCCM) is a standard for X Window System clients to interoperate on a single server. It was put together by the MIT X Consortium over 1988 and Version 1.0 was released in July 1989.
X deliberately specifies "mechanism, not policy." As such, a specification was needed for client interoperation. The ICCCM specifies cut and paste buffers, window manager interaction, session management, how to manipulate shared resources and how to manage device colours.
The ICCCM is notorious for having a difficult API [1]. Furthermore, some parts are obsolete or no longer practical to implement [2]. Most X programmers work to the specifications of their widget toolkit or desktop environment to achieve that level of inter-client communication. freedesktop.org and X.Org are working on updating the ICCCM to current needs.