Quoting Hendrik Boom (hendrik@???):

> My ancient experience with X was working on the team developing UIM/X in
> the days before Linux. I spent some time reading a manual caalled
> something like ICCCCC (maybe ICCCM?) about communications between these
> various network components, though I never had to do any coding directly
> with that communication.

ICCCM (Inter-Client Communication Conventions Manual).
https://www.x.org/releases/X11R7.6/doc/xorg-docs/specs/ICCCM/icccm.html

X deliberately specifies "mechanism, not policy" for how windows
interact. As such, an additional specification beyond the X protocol
itself 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. These low-level functions are generally
implemented within widget toolkits or desktop environments. This
isolates application programmers from working directly with the ICCCM
itself, as this functionality is delegated to the implementing toolkit.

The ICCCM is notorious for being ambiguous and difficult to correctly
implement. Furthermore, some parts are obsolete or no longer
practical to implement.

Efforts to update and clarify the ICCCM for current needs have resulted
in the Extended Window Manager Hints (EWMH), which has gained fairly
broad acceptance and continues to be extended as the need arises.

https://en.wikipedia.org/wiki/Inter-Client_Communication_Conventions_Manual

ICCCM came in for some memorable if passing derision in The UNIX-Hater's
Handbook. http://www.art.net/~hopkins/Don/unix-haters/x-windows/disaster.html

-- 
Cheers,                                       "The plural of regex is regrets."
Rick Moen                                      -- old coder gag, seen on Reddit
rick@???
McQ! (4x80)