A completely transparent window is easy to get -- use an InputOnly
window. In order to create a window which is *mostly* transparent, you have
several choices:
	- the SHAPE extension first released with X11R4 offers an easy way to
make non-rectangular windows, so you can set the shape of the window to fit the
areas where the window should be nontransparent; however, not all servers 
support the extension.
	- a machine-specific method of implementing transparent windows for
particular servers is to use an overlay plane supported by the hardware.  Note 
that there is no X notion of a "transparent color index".
	- a generally portable solution is to use a large number of tiny 
windows, but this makes operating on the application as a unit difficult.
	- a final answer is to consider whether you really need a transparent
window or if you would be satisfied with being able to overlay your application
window with information; if so, you can draw into separate bitplanes in colors
that will appear properly.

[thanks to der Mouse, mouse@lightning.McRCIM.McGill.EDU, 3/92; see also
The X Journal 1:4 for a more complete answer, including code samples for this
last option]