In General American, schwa is one of the two vowel sounds that can be rhotacized.
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I haven't seen any quotable sources on the development of schwa deletion, have you?
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Schwa is the third sound that most of the single vowel spellings can produce.
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Most common is the reduction of a vowel sound to a schwa when it becomes unstressed.
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I have been told the way to pronounce ain is like a schwa at the back of the throat.
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In the dialects of Catalan spoken in the Balearic Islands, a stressed schwa can occur.
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A more common usage of the schwa, however, is found in certain dialects or loanwords.
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In addition to the vowels listed above, schwa also occurs in Papiamento.
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In this case the term mid-central vowel may be used instead of schwa to avoid ambiguity.
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More examples
A neutral middle vowel; occurs in unstressed syllables
In linguistics, specifically phonetics and phonology, schwa (sometimes spelled shwa) can mean the following: *An unstressed and toneless neutral vowel sound in some languages, often but not necessarily a mid-central vowel. ...
Schwa is the underground conceptual artwork of Bill Barker (born 1957). Barker draws deceptively simple black and white stick figures and oblong alien ships. ...
Schwa (majuscule: , minuscule) is a letter of the Cyrillic alphabet. It is currently used in Abkhaz, Bashkir, Dungan, Kalmyk, Kazakh, Kurdish and Tatar. It was also used in Azeri and Turkmen before those languages switched to the Latin alphabet.
Schwa is an acclaimed, upscale restaurant run by chef-owner Michael Carlson in Chicago, USA. It is located in the West Town community area and is considered a leader in the emerging molecular gastronomy style of cooking. Schwa is known for its unconventional approach to business. ...
The Schwa Was Here is a young adult novel by Neal Shusterman. Published by Penguin Books and Dutton Books in 2004, it is about an eighth-grader's friendship with another student named Calvin Schwa, who is capable of seemingly not being noticed by the people around him.
Or shwa [?]: a vowel sound heard, for example, at the beginnings of the English words ago and amaze. In BC First Nations orthographies, this sound may be represented with the International Phonetic Alphabet letter ?, or by e, u, or other vowel symbols.
A short indeterminate vowel, like that at the end of "sofa". By far the commonest vowel sound in English.