In phonetics, stress is the degree of emphasis given a sound or syllable in speech, also called lexical stress or word stress. Unlike some other languages, English has variable (or flexible) stress. This means that stress patterns can help distinguish the meanings of two words or phrases that otherwise appear to be the same.
For example, in the phrase "every white house," the words white and house receive roughly equal stress; however, when we refer to the official home of the American president, "the White House," the word White is usually stressed more heavily than House.
These variations in stress account for the complexity of the English language, especially to those learning it as a second language. However, in all languages stress is used to make words more understandable on the word level and is especially apparent in the pronunciation of individual words and their parts.
Observations on Stress in Speech
Stress can be used to provide emphasis, but more often than not it is used to provide meaning to words in general and can be either associated word stress on the word, phrase or sentence levels.
Word-level stress, as Harold T. Edwards says in "Applied Phonetics: The Sounds of American English," is influenced by the context and content of the stress to inform meaning. He uses the example of two stresses of the word "record" to illustrate this point:
For example, We're going to record a record, the two similar words are stressed differently so that the first record is stressed on the second syllable (vowel reduction in the first syllable also assists in helping us to assign stress to the second syllable), whereas the second record is stressed on the first syllable (with vowel reduction in the second syllable). All words of more than one syllable have a prominent or stressed syllable. If we pronounce a word with appropriate stress, people will understand us; if we use the wrong stress placement, we run the risk of being misunderstood.
On the other hand, Edwards continues, phrase or sentence level stress is utilized in order to provide emphasis on a certain element of a given point, wherein phonetic stress focuses the audience's attention on what is most important in the message.
When linguistic changes occur through the gradual, varied use of a word or phrase in one region, especially as it relates to stressing words and phrases, a process known as lexical diffusion occurs; this is especially apparent in words that are used as both nouns and verbs, wherein the stress is changed between the different usages.
William O'Grady writes in "Contemporary Linguistics: An Introduction" that several such lexical diffusions have occurred since the last half of the sixteenth century. Words such as convert, he says, which can be used as either a noun or verb, drastically changed during this time. "Although the stress originally fell on the second syllable regardless of lexical category… three such words, rebel, outlaw, and record, came to be pronounced with the stress on the first syllable when used as nouns."
Thousands of other examples of the same exist, though O'Grady posits that not all have diffused through the entire English vocabulary. Still, words like report, mistake, and support give credence to this assumption, emphasizing the importance of stress in understanding spoken English.
Edwards, Harold T. "Applied Phonetics: The Sounds of American English." 3rd edition, Delmar Cengage, December 16, 2002.
O'Grady, William. "Contemporary Linguistics: An Introduction." John Archibald, Mark Aronoff, et al., Seventh edition, Bedford/St. Martin's, January 27, 2017.