18.4. Co-ordinate System.   

What is the size of the window we are using? GKS requires the user to specify the size of the window before using GKS primitives: default size is 0.0 -> 1.0, 0.0 -> 1.0. Similarly, the current viewport needs to be defined: defaults to 0.0 -> 1.0, 0.0 -> 1.0.

Since multiple windows and viewports are normal the window/viewport values are associated with an index.
i.e. set window (N, XWmin, XWmax, YWmin, YWmax)
set viewport (N, XVmin, XVmax, YVmin, YVmax)

This defines the Nth normalisation transformation = transformation of co- ordinate data into a window/viewport combination. Normally all such trans- formations are made at head of program and then
select normalisation transformation (n)
is used to enable that window/viewport combination.