Often explanations are cascaded (explained in terms of a lower-level frameworks). This effectively provides an open-ended how–why abstraction hierarchy. Note, however, that other forms of abstraction sometime play roles in explanations. For example, the elements of a system may themselves be decomposed as necessary into their molecules, atoms, elementary particles and quarks. Such structural decompositions are normally coordinated with how–why abstractions. Moreover, Simon's idea of nearly decomposable systems suggests that such breakdowns are often related to temporal abstractions. That is, events at lower levels typically occur at different frequencies from events at higher levels.