Stiffness and Rails

gentlemen,

Recent comments have piqued my curiosity re. the stiffness of various rail
systems. Let us say the rails are of adequate diameter or cross section in
the 3 axis; to what are these (x & y in particular ) rails mounted? I have
seen no mention in the list of the foundations or benches if you will, used
on such machines and not having any firsthand experience with them I wonder?
PJH, N7PXY