In computing, the Inter-Client Communication Conventions Manual (ICCCM or I39L short for "I", 39 letters and "L")[1] is a standard protocol for the X Window System.
Three window managers in the X11R4 contrib directory implemented support for ICCCM: gwm, olwm and tekwm.
This isolates application programmers from working directly with the ICCCM itself, as this functionality is delegated to the implementing toolkit.
[1][2] This was acknowledged in the preface of version 2.0, stating that it aimed "to fix problems with earlier drafts, and to improve readability and understandability".
Freedesktop.org notes that "historically, X clients have not handled cut-and-paste in a consistent way" and formulates additional guidelines in a clipboards-spec Archived 2021-02-17 at the Wayback Machine.