become, metaphorically, "in harmony"). The vowel that causes the vowel assimilation is frequently termed the trigger while the vowels that assimilate (or harmonize) are termed targets. When the vowel triggers lie within the root or stem of a word and the affixes contain the targets, this is called stem-controlled vowel harmony (the opposite situation is called dominant). This is fairly common among languages with vowel harmony and may be seen in the Hungarian dative suffix: {| cellspacing="5" ! Root ! Dative ! Gloss |- | | | 'city' |- | | | 'joy' |} The dative suffix has two different forms . The form appears after the root with back vowels ( and are back vowels). The form appears after the root with front vowels ( and are front vowels). Features of vowel harmony Vowel harmony often involves dimensions such as Nasalization (i.e. oral or nasal) (in this case, a nasal consonant is usually the trigger) In many languages, vowels can be said to belong to particular sets or classes, such as back vowels or rounded vowels. Some languages have more than one system of harmony. For instance, Altaic languages are proposed to have a rounding harmony superimposed over a backness harmony. Even among languages with vowel harmony, not all vowels need to participate in the vowel conversions; these vowels are termed neutral. Neutral vowels may be opaque and block harmonic processes or they may be transparent and not affect them. Intervening consonants are also often transparent. Finally, languages that do have vowel harmony often allow for lexical disharmony, or words with mixed sets of vowels even when an opaque neutral vowel is not involved. Van der Hulst & van de Weijer (1995) point to two such situations: polysyllabic trigger morphemes may contain non-neutral vowels from opposite harmonic sets and certain target morphemes simply fail to harmonize. Many loanwords exhibit disharmony. For example, Turkish , ('time' [from Arabic ]); * would have been expected. Languages with vowel harmony Korean There are three classes of vowels in Korean: positive, negative, and neutral. These categories loosely follow the front (positive) and mid (negative) vowels. Middle Korean had strong vowel harmony; however, this rule is no longer observed strictly in modern Korean. In modern Korean, it is only applied in certain cases such as onomatopoeia, adjectives, adverbs, conjugation, and interjections. The vowel () is considered a partially neutral and a partially negative vowel.