The first step of writing a new widget class is to decide the base class from which the new class. Usually, if the new class does not share any common features with other classes, it should be derived from the TixPrimitive class. If it does share common features with other classes, then it should be derived from the appropriate base class. For example, if the new class support scrollbars, it should be derived from TixScrolledWidget; if it displays a label next to its ``main area'', then it should be derived from TixLabelWidget.
In the case of our new TixArrowButton class, it doesn't really share any common features with other classes, so we decide to use the base class TixPrimitive as its superclass.