Several difficulties have been identified with these systems of identifying oral narratives according to either tale types or story elements. A first major problem is their hierarchical organisation; one element of the narrative is selected as the major element, while all other parts are arrayed subordinate to this. A second problem with these systems is that the listed motifs are not qualitatively equal; actors, items and incidents are all considered side-by-side. And because incidents will always have at least one actor and usually have an item, most narratives can be ordered under multiple headings. This leads to confusion about both where to order an item and where to find it. A third significant problem is that the "excessive prudery" common in the middle of the 20th century means that obscene, sexual and scatological elements were regularly ignored in many of the indices.