Stress is a prominent feature of the English language, both at the level of the word (lexical stress) and at the level of the phrase or sentence (prosodic stress). Absence of stress on a syllable, or on a word in some cases, is frequently associated in English with vowel reduction – many such syllables are pronounced with a centralized vowel (schwa) or with certain other vowels that are described as being "reduced" (or sometimes with a syllabic consonant as the syllable nucleus rather than a vowel). Various phonological analyses exist for these phenomena.
For example, in the following sentence, a speaker would typically pronounce have with a schwa, as /həv/ or /əv/ (homophonous with of):
Alice and Bob have arrived.
But in other contexts where the word carries stress, it would be pronounced in its "strong" (unreduced) form as /hæv/ (homophonous with halve). For example:
Alice and Bob have three children. [In response to the question "Have Alice and Bob arrived?"] They have.
Lexical and prosodic stress
Lexical stress (word stress) is regarded as being phonemic in English; the position of the stress is generally unpredictable and can serve to distinguish words. For example, the words insight and incite are distinguished in pronunciation only by the syllable being stressed. In insight, the stress is placed on the first syllable; and in incite, on the second. Similarly, the noun and the verb increase are distinguished by the placement of the stress in the same way – this is an example of an initial-stress-derived noun.
Moreover, even within a given letter sequence and a given part of speech, lexical stress may distinguish between different words or between different meanings of the same word (depending on differences in theory about what constitutes a distinct word): for example, initial-stress pronunciations of offense and defense in American English denote concepts specific to sports, whereas pronunciations with stress on the words' respective second syllables (offense and defense ) denote concepts related to the legal (and, for defense, the military) field, and encountered in sports only as borrowed from the legal field in the context of adjudicating rule violations. British English stresses the second syllable in both sports and legal use.
Some words are shown in dictionaries as having two levels of stress: primary and secondary. For example, the RP pronunciation of organization may be given as , with primary stress on the fourth syllable, secondary stress on the first syllable, and the remaining syllables unstressed. For different ways of analysing levels of stress in English, see below.
English also has relatively strong prosodic stress—particular words within a phrase or sentence receive additional stress to emphasize the information they convey. There is also said to be a natural "tonic stress" that falls on the last stressed syllable of a prosodic unit – for more on this, see below under .
English is classified as a stress-timed language, which means that there is a tendency to speak so that the stressed syllables come at roughly equal intervals. See .
Reduced vowels
Certain vowel sounds in English are associated strongly with absence of stress: they occur practically exclusively in unstressed syllables; and conversely, most (though not all) unstressed syllables contain one of these sounds. These are known as reduced vowels, and tend to be characterized by such features as shortness, laxness and central position. The exact set of reduced vowels depends on dialect and speaker; the principal ones are described in the sections below.
Schwa and r-coloured schwa
Schwa, , is the most common reduced vowel in English. It may be represented in spelling by any of the vowel letters, such as the a in about, the o in harmony, the u in medium, the e in synthesis, the i in decimal or the y in analysis (although the last three are pronounced instead as a near-close vowel by some speakers – see the following section).
In many rhotic dialects, an r-colored schwa, , occurs in words such as water and standard. Non-rhotic dialects simply have schwa in these positions, except where the dialect has linking R (although when it is coupled with intrusive R, the underlying phoneme is still a bare , removing any phonemic difference). The r-colored schwa can be analyzed phonemically as .