The top level is the root of the folklore its self ... for example "Irish Folklore" would have a root simply named "Irish"
Folklore is full of a lot of things from characters, to places, species, beasties, artefacts and even complex concepts such as Fait or Destiny.
This is the discreet subjects its self, a specific character, a particular place, creature or tribe
Most things evolve over time and folklore doesn't happen over night. As such many "subjects" will have evolved from some early proto form to some later echo of that idea. The evolution section is a container that houses each period in that subjects evolution.
Contained within the Evolution subjection, each subject may have many periods. The periods each subject has will depend on that subject.
Some subjects may be a spin off of other subjects. ... that is a subject may fragment into later distinct subjects and similarly we see some subjects merge together to become more unified subjects in later periods
The period system is capable of being stand alone articles or links to other subjects as need demands.