Checked and free vowels

In phonetics and phonology, checked vowels are those that commonly stand in a stressed closed syllable, while free vowels are those that can stand in either a stressed closed syllable or a stressed open syllable.

Usage
The terms checked vowel and free vowel originated in English phonetics and phonology; they are seldom used for the description of other languages, even though a distinction between vowels that usually have to be followed by a consonant and other vowels is common in most Germanic languages.

The terms checked vowel and free vowel correspond closely to the terms lax vowel and tense vowel respectively, but linguists often prefer to use the terms checked and free, as there is no clearcut phonetic definition of vowel tenseness and because by most attempted definitions of tenseness and  are considered lax, even though they behave in American English as free vowels.

Checked vowels is also used to refer to the kind of very short glottalized vowels heard in the Zapotecan languages of Oaxaca, Mexico, that contrast with laryngealized vowels. The term checked vowel is also used to refer to a short vowel followed by a glottal stop in Mixe, which has a distinction between two kinds of glottalized syllable nuclei: checked ones, with the glottal stop after a short vowel, and nuclei with rearticulated vowels, a long vowel with a glottal stop in the middle.

English
In English, the checked vowels are the following:
 * as in pit
 * as in pet
 * as in pat
 * as in pot (in varieties without the cot–caught merger or the father–bother merger)
 * as in put (in varieties without the foot–goose merger)
 * as in putt

There are a few exceptions, mostly in interjections: eh and meh with ; duh, huh, uh, uh-uh, and uh-huh with ; nah with or ; and yeah with  (in accents that lack the diphthong ) or. There are also the onomatopoeia baa for and the loanword pho for  when pronounced in American English, as well as sometimes milieu and pot-au-feu. The proper names Graham and Flaherty are sometimes pronounced with a prevocalic.

The free vowels are the following:
 * as in pea
 * as in pay
 * as in too
 * as in Poe
 * as in paw
 * as in bra
 * as in pie
 * as in plow
 * as in ploy

The schwa is usually considered neither free nor checked because it cannot stand in stressed syllables.

In non-rhotic dialects, non-prevocalic instances of as in purr, burr and  as in letter, banner pattern as vowels, with the former often being the long counterpart of the latter and little to no difference in quality:. In rhotic dialects, they pattern as vowel+consonant sequences, following the historical situation, even though they often surface as rhotacized vowels: (or, in other analyses, syllabic postalveolar/retroflex approximants:  etc.)

The same applies to, and , which are realized as centering diphthongs or long monophthongs in non-rhotic varieties of English, but as vowel+consonant sequences (alternative analysis: centering diphthongs with a rhotacized offset) in rhotic English.

The term checked vowel is also useful in the description of English spelling. As free written vowels a, e, i, o, u correspond to the spoken vowels, , , , ; as checked vowels a, e, i, o, u correspond to , , , ,. In spelling free and checked vowels are often called long and short, based on their historical pronunciation, though nowadays some or all of the free vowels are diphthongs, depending on the dialect, not long vowels as such. Written consonant doubling often shows the vowel is checked; the i of dinner corresponds to checked because of the double consonants nn; the i of diner corresponds to free  because of the single consonant n. This, however, interferes with the differences in doubling rules between American and British styles of spelling. Similarly, an "e" following a single consonant at the end of a word often indicates that the preceding vowel is free where it would otherwise be checked; for example, the a of tap corresponding to whereas that in tape corresponds to.