In English, an unstressed or reduced vowel is the vowel sound that forms the syllable peak of a syllable that has no lexical stress. This sound is typically a schwa, although there are other vowels that can be unstressed or reduced. An reduced vowel is one of the vowels that can only occur in unstressed syllables, like schwa, and an unstressed vowel is one of the vowels that can be stressed but is not.

Reduced vowels

Schwa is the most common reduced vowel in English, and may be denoted by any of the vowel letters:

• The a in about is a schwa.
• The e in synthesis is a schwa.
• The i in decimal is schwa, except in dialects that have two distinct reduced vowels (see below).
• The o in harmony is a schwa.
• The u in medium is a schwa.
• The y in syringe is a schwa.

Whereas the sound represented by the er in water is a schwa in non-rhotic accents like Received Pronunciation, in rhotic dialects like most of North American English, this sound is not a schwa sound; rather, the "er" designates an r-colored schwa, , which is pronounced like schwa, except the tongue is pulled back in the mouth and "bunched up".

In some dialects of English there is a distinction between two vowel heights of reduced vowels, schwa and barred i, the close central unrounded vowel /ɨ/. In the British phonetic tradition, /ɪ/ is used to transcribe this vowel in British English instead of /ɨ/, but the sound is the same. An example of a minimal pair contrasting schwa and barred i:

• The e in roses is a barred i
• The a in Rosa's is a schwa

The other sounds that can serve as the peak of reduced syllables are the syllabic consonants . The consonants that can be syllabic in English are the nasals /m/, /n/, /ŋ/, and the dark l /ɫ/. For example:

• The m in prism is sometimes a syllabic /m/.
• The on in button is a syllabic /n/ in dialects that pronounce the 'tt' as a glottal stop.
• The word and in the phrase lock and key is sometimes pronounced as a syllabic /ŋ/.
• The le in cycle and bottle is a syllablic /ɫ/.

Unstressed vowels

All the other vowels in English can occur in unstressed syllables, although whether a unreduced vowel in such a syllable is really unstressed or merely has secondary stress is debatable. Unstressed [i] and [u] are sometimes considered separately from the other unstressed vowels and are called schwi and schwu, respectively.

For example: