Schwa, the vowel with no letter of its own

The commonest English vowel has no separate letter to represent it in the English alphabet. The vowel is called ‘schwa’ and is found as, for example, the first vowel in about.

Schwa is a mid central unrounded vowel:

• mid means that the highest part of the tongue is neither high (where it is for /i/ and /u/) nor low (where it is for /a/ and /ɑ/).
• central means that the highest part of the tongue is at neither the front of the mouth (where it is for /i/, /e/, /ɛ/ and /a/) nor the back of the mouth (where it is for /u/, /o/, /ɔ/ and /ɑ/).
• unrounded means that the lips are not rounded. (the lips are rounded for /u/, /o/, /ɔ/ and /ɑ/)

The position of the tongue for schwa and other (‘cardinal’) vowels is shown in diagram 1, originally from How mouth shape affects vowel sounds – Language Miscellany

Neutral vowel

In a sense, schwa is the neutral vowel because the highest part of the tongue is in the middle of the mouth, not near one edge, and because the lips are in a relaxed position. Said differently, schwa carries none of the features that tend to distinguish other vowels, for example [front], [back], [high], [low], [rounded], [nasal].

So, not surprisingly, schwa tends to arise:

• as a default when an unspecified vowel is inserted to, for example, break up a cluster of consonants; or
• when there is no need to emphasise that vowel.

Stress

Wells (2014) says that in English schwa is typically unstressed (unless the vowel in, for example, luck, is transcribed as /ə/ rather than /ʌ/.) He says that English schwa is occasionally ‘restressed’ (a term he does not explain) in forms such as (be)cause [kəz], just [dʒəst] and gonna [gənə] (contraction of going to).

Wells gives as examples of languages in which schwa can be stressed Welsh (eg mynydd, /mənɪd/ ‘mountain’, with stressed 1^st syllable), Romanian (eg măr /mər/ ‘apple’ tree’) and Bulgarian (eg съм /səm/ ‘I am’).

Frequency

Schwa is the most common English vowel. According to a 1947 study reported in Cruttenden (2008, 7^th ed156), schwa accounts for 27% of all English vowels in colloquial ‘Received Pronunciation’ (RP).

The only other comparably common vowel was /ɪ/ (21%), which is often unstressed but sometimes stressed. The next most common vowel was /e/ and accounted for only 8% of the total. According to Cruttenden, other studies show similar results, for both British and American English.

Schwa symbol

The International Phonetic Alphabet (IPA) does have a symbol for schwa: /ə/.

Wells points out that at least one language—the Turkic language Azeri, also known as (North) Azerbaijani—uses the schwa symbol in its normal spelling. In Azeri, though, the schwa symbol denotes not the IPA mid central vowel /ə/ but the front open vowel /æ/ (as in standard southern British English trap).  

Phoneme or allophone?

Wells and Giegerich both discuss whether schwa is a separate phoneme in English, or just an unstressed allophone of all stressed vowels. Neither of them seems to come to a clear conclusion.

Their reluctance to reach a conclusion seems to rest largely on the fact that /ɪ/ can sometimes appear in the same environment as schwa, with the distinction between the 2 forms varying (with some instability) between different accents and different speakers.

For more on the distinction between phonemes and allophones, please see Phonemes and allophones – Language Miscellany

Sources

Gimson’s Pronunciation of English, revised by Alan Cruttenden (7th edition, 2008)

Sounds Interesting: Observations on English and General Phonetics, JC Wells (2014)

English Phonology: an Introduction, Heinz J Giegerich (1992)