In linguistics, and particularly phonology, stress or accent is the relative emphasis or prominence given to a certain syllable in a word or to a certain word in a phrase or sentence. That emphasis is typically caused by such properties as increased loudness and vowel length, full articulation of the vowel, and changes in tone. The terms stress and accent are often used synonymously in that context but are sometimes distinguished. For example, when emphasis is produced through pitch alone, it is called pitch accent, and when produced through length alone, it is called quantitative accent. When caused by a combination of various intensified properties, it is called stress accent or dynamic accent; English uses what is called variable stress accent. Since stress can be realised through a wide range of phonetic properties, such as loudness, vowel length, and pitch (which are also used for other linguistic functions), it is difficult to define stress solely phonetically. The stress placed on syllables within words is called word stress. Some languages have fixed stress, meaning that the stress on virtually any multisyllable word falls on a particular syllable, such as the penultimate (e.g. Polish) or the first (e.g. Finnish). Other languages, like English and Russian, have lexical stress, where the position of stress in a word is not predictable in that way but lexically encoded. Sometimes more than one level of stress, such as primary stress and secondary stress, may be identified. Stress is not necessarily a feature of all languages: some, such as French and Mandarin Chinese, are sometimes analyzed as lacking lexical stress entirely. The stress placed on words within sentences is called sentence stress or prosodic stress. That is one of the three components of prosody, along with rhythm and intonation. It includes phrasal stress (the default emphasis of certain words within phrases or clauses), and contrastive stress (used to highlight an item, a word or part of a word, that is given particular focus).
Phonetic realization There are various ways in which stress manifests itself in the speech stream, and they depend to some extent on which language is being spoken. Stressed syllables are often louder than non-stressed syllables, and they may have a higher or lower pitch. They may also sometimes be pronounced longer. There are sometimes differences in place or manner of articulation. In particular, vowels in unstressed syllables may have a more central (or "neutral") articulation, and those in stressed syllables have a more peripheral articulation. Stress may be realized to varying degrees on different words in a sentence; sometimes, the difference is minimal between the acoustic signals of stressed and those of unstressed syllables. Those particular distinguishing features of stress, or types of prominence in which particular features are dominant, are sometimes referred to as particular types of accent: dynamic accent in the case of loudness, pitch accent in the case of pitch (although that term usually has more specialized meanings), quantitative accent in the case of length, and qualitative accent in the case of differences in articulation. They can be compared to the various types of accents in music theory. In some contexts, the term stress or stress accent specifically means dynamic accent (or as an antonym to pitch accent in its various meanings). A prominent syllable or word is said to be accented or tonic; the latter term does not imply that it carries phonemic tone. Other syllables or words are said to be unaccented or atonic. Syllables are frequently said to be in pretonic or post-tonic position, and certain phonological rules apply specifically to such positions. For instance, in American English, /t/ and /d/ are flapped in post-tonic position. In Mandarin Chinese, which is a tonal language, stressed syllables have been found to have tones that are realized with a relatively large swing in fundamental frequency, and unstressed syllables typically have smaller swings. (See also Stress in Standard Chinese.) Stressed syllables are often perceived as being more forceful than non-stressed syllables.
Word stress Word stress, or sometimes lexical stress, is the stress placed on a given syllable in a word. The position of word stress in a word may depend on certain general rules applicable in the language or dialect in question, but in other languages, it must be learned for each word, as it is largely unpredictable, for example in English. For instance, in English, stress falls on the middle syllable in the word projector but on the first syllable in the word property. In some cases, classes of words in a language differ in their stress properties; for example, loanwords into a language with fixed stress may preserve stress placement from the source language, or the special pattern for Turkish placenames.
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