In phonology, vowel harmony is an assimilatory process in which the vowels of a given domain – typically a phonological word – have to be members of the same natural class (thus "in harmony"). Vowel harmony is typically long distance, meaning that the affected vowels do not need to be immediately adjacent, and there can be intervening segments between the affected vowels. Generally one vowel will trigger a shift in other vowels, either progressively or regressively, within the domain, such that the affected vowels match the relevant feature of the trigger vowel. Common phonological features that define the natural classes of vowels involved in vowel harmony include vowel backness, vowel height, nasalization, roundedness, and advanced and retracted tongue root. Vowel harmony is found in many agglutinative languages. The given domain of vowel harmony taking effect often spans across morpheme boundaries, and suffixes and prefixes will usually follow vowel harmony rules. Terminology The term vowel harmony is used in two different senses. In the first sense, it refers to any type of long distance assimilatory process of vowels, either progressive or regressive. When used in this sense, the term vowel harmony is synonymous with the term metaphony. In the second sense, vowel harmony refers only to progressive vowel harmony (beginning-to-end). For regressive harmony, the term umlaut is used. In this sense, metaphony is the general term while vowel harmony and umlaut are both sub-types of metaphony. The term umlaut is also used in a different sense to refer to a type of vowel gradation. This article will use "vowel harmony" for both progressive and regressive harmony. "Long-distance" Harmony processes are "long-distance" in the sense that the assimilation involves sounds that are separated by intervening segments (usually consonant segments). In other words, harmony refers to the assimilation of sounds that are not adjacent to each other. For example, a vowel at the beginning of a word can trigger assimilation in a vowel at the end of a word. The assimilation occurs across the entire word in many languages. This is represented schematically in the following diagram: {| cellspacing="5" style="font-size: 110%;" ! beforeassimilation | ! afterassimilation | |- | VaCVbCVbC | → | VaCVaCVaC | (Va = type-a vowel, Vb = type-b vowel, C = consonant) |} In the diagram above, the Va (type-a vowel) causes the following Vb (type-b vowel) to assimilate and become the same type of vowel (and thus they