Back vowels are vowel sounds produced with the highest point of the tongue body positioned toward the back of the mouth, relative to front and central vowels.[1]
Back vowels are widespread cross-linguistically. In the PHOIBLE 2.0 convenience sample of phonological inventories, [u] and [o] are among the most frequently listed vowel segments.[2][3] As displayed on PHOIBLE Online’s segment summary, [u] appears in 88% of inventories and [o] appears in 60% (counts over PHOIBLE’s inventory sample).[4]
Back vowels are often—but not always—rounded. A common typological pattern is that non-low back vowels are rounded (e.g. [uo]), while low vowels are often unrounded; front rounded vowels and back unrounded vowels also occur but are less common.[5]
Transcription conventions
In strict IPA usage, the open front unrounded vowel is [a] and the open back unrounded vowel is [ɑ]. However, many phonological traditions use [a] (or the letter ⟨a⟩) more loosely for a low vowel, and recommend clarifying front vs. central/back values when the distinction is important.[6] In descriptions of backness-based vowel harmony, a low vowel written /a/ may pattern with the back-vowel set even when its phonetic realization is not narrowly transcribed as [ɑ].[7]
Phonology
Vowel backness is often treated as a distinctive feature (commonly [±back]) and can participate in processes such as vowel harmony, where vowels within a domain (often the word) systematically agree for backness and sometimes other properties.[7][8] Back vowels form a class defined by tongue-body retraction, but they may differ in vowel height (e.g. close [u] vs. open [ɑ]) and rounding; intermediate degrees of retraction and centralization can be transcribed with IPA diacritics for relative advancement or retraction and centralization (e.g. ⟨u̟⟩, ⟨u̠⟩, ⟨ü⟩), and some phonetic descriptions further distinguish types of rounding (e.g. protrusion vs. labial compression).[1][9]
Acoustics and perception
The vowel diagram used in IPA description is primarily a reference space for perceived vowel quality and does not directly encode vocal-tract shapes.[10] Acoustically, perceived vowel backness is correlated (roughly) with the second formant (F2), with back vowels tending to have lower F2 values than front vowels.[10] Lip rounding lowers vocal-tract resonances and can reinforce the acoustic profile associated with back vowels, which has been argued to support the common co-occurrence of backness and rounding.[5]
Partial list
Back (and near-back) vowels with dedicated IPA symbols include:
12International Phonetic Association (1999). Handbook of the International Phonetic Association: A Guide to the Use of the International Phonetic Alphabet. Cambridge University Press. pp.11–13. ISBN9780521637510.
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