This section gives technical details that weren't covered in the Intermediate section, about sounds that were covered.
This feature is common in agglutinative languages, i.e. in languages where one suffix represents one property (Turkic, Mongolian, Finnish, Hungarian). All vowels are divided in two groups (back vowels and front vowels), and only vowels from one group can be used in the word; therefore all suffices have two forms.
For example, in Tatar language the groups are following:
First group (Back vowels): a, ı, í, o, u Second group (Front vowels): ä, e, i, ö, ü
Plural suffix has forms lar and lär. The word for friend is dus, so friends is duslar; the word for house is öy, therefore, houses is öylär.
Sometimes there are neutral vowels that belong to both groups, like i and e in Finnish:
First group (Back vowels): a, o, u, e, i Second group (Front vowels): ä, ö, y, e, i
Usually group is selected by the kind of vowels in the root of the word. But in Chukchi language, for example, there are dominant and recessive group, and all vowels from recessive group are replaced with their dominant counterparts when a suffix with a dominant vowel is used.