"Sources Of Geometry Information"
Size-related information for top-level windows
can come from three sources.
First, geometry requests come from the widgets that are descendants
of a top-level window.
Each widget requests a particular size for itself
by calling Tk_GeometryRequest. This information is passed to
geometry managers, which then request large enough sizes for parent
windows so that they can layout the children properly.
Geometry information passes upwards through the window hierarchy
until eventually a particular size is requested for each top-level
These requests are called internal requests in the discussion
The second source of width and height information is through the
wm :geometry command. Third, the user can
request a particular size for a window using the
interactive facilities of the window manager.
The second and third types of geometry requests are called
external requests in the discussion below; Tk treats
these two kinds of requests identically.