The Korean language of this period had vowel harmony to a greater extent than it does today. Vowels alternated according to their environment, and fell into "harmonic" groups. This affected the morphology of the language, and Korean phonology described it in terms of yin and yang: If a word had yang ('bright') vowels, then most suffixes also had to have a yang vowel; and conversely, if the root had yin ('dark') vowels, the suffixes needed to be yin as well. There was a third group called "mediating" ('neutral' in Western terminology) that could coexist with either yin or yang vowels.
