ICCCM

ICCCM (Inter-Client Communications Conventions Manual) is a set of standards regulating the functionality of window managers under X11. You can read the specification for ICCCM here: http://tronche.com/gui/x/icccm/

ICCCM has lately been extended by a new specification, EWMH (Extended Window Manager Hints) a.k.a. NetWM. Certain parts of ICCCM are superseded by parts of NetWM.