A parent container. When a widget is created it can be incorporated into the widget heirarchy by passing in a parent container at construction time. (For some toolkits this is not optional, e.g. gWidgets2tcltk or gWidgets2WWW2.)

These values are passed to the add method of the parent container, and occasionally have been used to sneak in hidden arguments to toolkit implementations.

Each widget constructor is passed in the toolkit it will use. This is typically done using the default, which will lookup the toolkit through guiToolkit.