The class Packer.html#ContainerContainer designates GUI elements into which other GUI elements may be packed.
Instances of Container include Toplevel (windows), HTk (Tk's root window), Frame, and furthermore Canvas, and Editor (and a few Tix widgets).
The class Container is abstract -- it has no class functions, and only serves to structure the code. Abstract classes are used frequently in HTKto impose a typing discipline onto Tk's untyped GUI element structure, with the benefit that type checking can prevent run time errors.