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:
As a consequence Finns often have trouble pronouncing foreign loanwords which violate these rules. The word "Olympia" for example, tends to become "Olumpia" in their mouths.
In Mongolian, the rule states:
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.
The counterpart of vowel harmony, consonant harmony, is less widespread. Most commonly, consonant harmony requires all the sibilants of the word to belong either to the anterior class (s-like sounds) or the nonanterior class (sh-like sounds). Such patterns are found in Navajo, Kinyarwanda, and elsewhere. Various Austronesian languages exhibit consonant harmony among the liquids, with [r] assimilating at a distance to [l] or vice versa.