Vertical vowel system refers to a system of vowels in a language which uses just one vowel dimension to phonemically distinguish vowels. Theoretically, rounding, frontness and backness , and vowel height could be used in one-dimensional vowel systems; however, vertical refers to the usage of vowel height as the sole distinguishing feature.

Vertical vowel systems need not be phonetically vertical and, in fact, rarely are. Wichita, for instance, possesses a vertical vowel system; however, two of the vowels it uses (e and i) are phonetically fronted, and the other (a) is phonetically backed. o is also heard phonetically, but is a contraction of [short vowel + w + short vowel]. More than twenty different vowel shadings have been noted in Kabardian, and in Ubykh, complex phonetic vowel features such as nasality have been noted.

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 have been noted for the following languages:

• Northwest Caucasian family
  • Abkhaz (distinguishes a and i)
  • Adyghe (distinguishes a, e and i)
  • Kabardian (distinguishes a, i and possibly e)
  • Ubykh (distinguishes a and i)
• Caddoan family
  • Wichita (distinguishes a, e and i)
• Australian Aboriginal group
  • Arrernte