HOW-TO write an X window manager

Please excuse me if this question should be directed to a different 
list. I'm new to X and still exploring.

Maybe I'm looking for the wrong thing, but I can't seem to find any 
information on how to write a window manager for X. I can and will look 
at the code for existing ones, but they'll only show me what those 
window managers do -- they won't describe the range of what's possible.

Is there a spec somewhere that describes what it means to be a window 
manager or a how to guide for creating one?

Many thanks for any guidance you can provide.