[incr Tk]

[incr Tk] is a paradigm for building mega-widgets, and a set of base classes which support the paradigm. The inheritance hierarchy for [incr Tk] looks like this:
[incr Tk] supplies three base classes that reside in the itk namespace: All [incr Tk] mega-widgets are derived from itk::Archetype, which manages component widgets and configuration options. The base class itk::widget adds a frame which acts as the "hull" for the mega-widget. Mega-widgets like a Combobox might be derived from itk::Widget, since they sit inside of other frames and toplevels. The base class itk::Toplevel, on the other hand, adds a toplevel widget as the "hull" component. Mega-widgets like a Dialog or MessageDialog might be derived from itk::Toplevel, since they sit in their own toplevel window.

Mega-widget designers specialize these base classes by adding components into the "hull". The [incr Widgets] package, for example, is built in exactly this manner. Here are some more examples: