Back vowels arevowel sounds articulated with the primary tongue constriction (the highest point of the tongue body) located toward the back of the oral cavity, near thevelum, relative tofront andcentral vowels.[1]
Back vowels are common across languages. In the PHOIBLE 2.0 convenience sample of phonological inventories,[u] and[o] are among the most frequently listed vowel segments, occurring in 88% and 60% of inventories in the sample, respectively.[2][3] Back vowels are often, but not always,rounded: cross-linguistically, non-low back vowels are commonly rounded (e.g.[uo]), while low vowels are often unrounded; front rounded vowels and back unrounded vowels also occur, but are less common.[4]
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.[5]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].[6]
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.[6][7] Back vowels form a class defined by tongue-body retraction, but they may differ invowel height (e.g. close[u] vs. open[ɑ]) androunding; 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][8]
The vowel diagram used in IPA description is primarily a reference space for perceived vowel quality and does not directly encode vocal-tract shapes.[9]Acoustically, perceived vowel backness is correlated (roughly) with the second formant (F2), with back vowels tending to have lower F2 values than front vowels.[9]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.[4]
Back (and near-back) vowels with dedicated IPA symbols include:
Vowels without dedicated IPA letters can be transcribed using diacritics forrelative articulation (e.g. ⟨o̞⟩, ⟨u̠⟩, ⟨u̜⟩).[10]
