Two different diachronic mechanisms may give rise to a vertical vowel system. In some cases, a the front-back distinction may simply be lost when vowels are merged. This has occurred in Wichita, in which an old vowel /u/ (preserved in the related language Pawnee) has merged with /i/. However, the Wichita vowel system is not phonetically vertical, as /a/ is realised as open back, /e/ as open-mid front, and /i/ as close to close-mid front; hence, the feature [± back] is relevant to the phonetics of the language, even though it is not a salient phonological distinction. Similarly, the vowel [o] is heard in Wichita utterances, although this vowel is usually the phonetic result of a contraction of sequences of [short vowel + w + short vowel], a phenomenon also noted in other languages with vertical vowel systems.
More striking is a phenomenon whereby one or more phonological features of vowels are lost and reassigned to the consonants at the syllable periphery, leaving all vowels underspecified for frontness, rounding, or both. This has occurred in Arrernte, in which vowel rounding has been lost and consonantal labialisation gained as a result; famously, in all members of the Northwest Caucasian family, both rounding and frontness have been reassigned to the syllable periphery, the former surfacing as consonantal labialisation, and the latter as palatalisation. This has also occurred in Marshallese. Some argue that the short vowels of Irish have similarly lost their frontness specification, forming a rudimentary vertical system. However, almost all Irish consonants appear in palatalised and non-palatalised forms, so the loss of frontness specification is viewed as a consequence, rather than a cause, of consonant palatalisation. Furthermore, the loss of frontness specification in Irish is limited to the short vowels of the language; the long vowels of Irish retain a front-back distinction.
Zero-dimensional vowel systems (one phonemic vowel only) have been postulated for some Abkhaz dialects, and for Kabardian; however, it is generally accepted that these analyses are flawed.
Vertical vowel systems, invariably contrasting only in vowel height, have been noted for the following languages: