In phonetics and phonology, checked vowels are those that usually must be followed by a consonant in a stressed syllable, while free vowels are those that may stand in a stressed open syllable with no following consonant.
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 those that do not have to 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 many linguists prefer to use the terms checked and free as there is no clearcut phonetic definition of vowel tenseness, and since by most attempted definitions of tenseness /ɔː/ and /ɑː/ are considered lax, even though they behave in American English as free vowels.
Checked vowels is also used to refer to a kind of very short glottalized vowels found in some Zapotecan languages 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, where there is 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).
In General American, the five checked vowels are:
There are a few exceptions, mostly in onomatopoeias: yeah /jæ/; eh /ɛ/; duh, huh, uh, uh-uh, and uh-huh with /ʌ/.
The free vowels are:
The schwa /ə/ and rhotacized schwa /ɚ/ are usually considered neither free nor checked, since they cannot stand in stressed syllables at all.