The 'Screen' is an Xlib structure which includes the information about
one of the monitors or virtual monitors which a single X display supports. A 
server can support several independent screens. They are numbered unix:0.0,
unix:0.1, unix:0.2, etc; the 'screen' or 'screen_number' is the second digit --
the 0, 1, 2 which can be thought of as an index into the array of available 
Screens on this particular Display connection.
	The macros which you can use to obtain information about the particular
Screen on which your application is running typically have two forms -- one
which takes a Screen and one with takes both the Display and the screen_number.
	In Xt-based programs, you typically use XtScreen(widget) to determine 
the Screen on which your application is running, if it uses a single screen.
	(Part of the confusion may arise from the fact that some of the macros
which return characteristics of the Screen have "Display" in the names -- 
DisplayWidth, DisplayHeight, etc.)