Vowel harmony

In linguistics, a language exhibiting vowel harmony has a phonological rule that requires all vowels in a word to belong to a single class of vowels. The most common types of vowel harmony rules are rules requiring all vowels to be either rounded or unrounded, or requiring all vowels to be either front or back vowels.

For example, in the Finnish language, there are three classes of vowels:

1. Front vowels: (/ü/, /ö/, /æ/)
2. Back vowels: (/u/, /o/, /a/)
3. Neutral vowels: (/i/, /e/) [non-low unrounded vowels]

And the rule states that words may contain vowels from group 3 and/or vowels from either group 1 or group 2 (but not both 1 and 2). Thus, [tütö], [tütöti], and [tutoti] are permissible, but *[tutöti] and *[tüto] are not.

In Mongolian, the rule states:

1. Front vowels: (/e/, /ö/, /ü/)
2. Back vowels: (/a/, /o/, /u/)
3. Neutral vowel: (/i/)

Other languages, such as Korean, have more arbitrary class-membership rules. This phenomenon has been documented in Telugu, several Bantu languages, and almost all languages in the Ural-Altaic language family.

Compound words often violate this rule; for instance the Finnish month name "syyskuu" ("kuu" means month). In such words suffixes agree with the vowels in the last part: syyskuuta.

See also: Uralic languages, Hungarian, Altaic languages, Turkic languages, Turkish language.

Note: There are no known languages that exhibit consonant harmony in this fashion. However, assimilation of consonants and consonant cluster simplification could be taken to be cousins of vowel harmony.