The commonly accepted explanation for the hierarchy outlined in the previous section is that the types of relation at the top of the deranking hierarchy are much more semantically integrated than those at the bottom. Being semantically integrated means that the events in the main and subordinate clauses are linked, which is true of purpose, perception, "before", "when" and "after" clauses, but not of those further rightward in the hierarchy. This integration leads to the use of verb forms not marked for tense, person or aspect, since they are much simpler than verb forms with these markers.