This tendency that vowels were divided into groups and often combined with other vowels of the same group and rarely with the ones of other groups was considered as a trace of 'vowel harmony' observed in Turkish for example.
母音同士が共存しやすいグループを作り、互いに同グループの母音と共存しやすく他グループの母音とは共存しにくいという傾向はトルコ語などに見られる「母音調和」現象の名残とされた。