In Opal, the hierarchy among topics is described metaphorically in terms of generational relationships among females.
An ancestress of a topic is any topic whose family includes it. Thus, starting with any topic not at top level, its mother is an ancestress of it, and so is the mother of that topic, and the mother of that topic, and so on.
An Opal outline may optionally be set to have a single ancestress. This means that there is only one top-level topic, and all other topics have that single top-level topic as their ancestress.