koreanhistory1-3, Exercise 3: The bone-rank system (a sample; yours will differ)

Model answer:

True bone (jingol): could hold any post up to full minister and office rank up to ibolchan, the highest; after the sacred-bone rank ended, only true-bone holders could become king. They could not marry into lower ranks, though they could take lower-ranked concubines.

Head rank 6: could be vice-minister (gyeong) and reach office rank achan (the sixth of 17 levels); not the top ranks of office, and not kingship.

Why not fixed from the start: the article says the origins of the lower head ranks are "obscure and still debated", and gives two views of its establishment (Lee Ki-baik: adopted from Chinese administrative law, promulgated by King Beopheung in 520; McBride: it emerged in the sixth century as Silla absorbed neighbouring peoples). Also, the sacred-bone rank was abolished under Muyeol, so the ladder itself changed over time.