Usually, both rectangles are aligned with the coordinate axes.
\
Window has corners and
;
Viewport has corners and
.
Length and height of the window are and
,
Length and heigth of the viewport are and
.
and similarily for .
Readings: Hearn and Baker: Section 6-3.
Red book: 5.5.
White book: 5.4,
Blinn: 16.
Intuitively, the window-to-viewport formula can be read as: