Basic Grammar: What is a Diphthong?

Vowel Sounds that Morph in a Single Syllable

diphthong
In most dialects of English, the vowel sounds in these words are diphthongs. Illustration by Claire Cohen. 2018 ThoughtCo. 

The word "diphthong" comes from the Greek and means "two voices" or "two sounds." In phonetics, a diphthong is a vowel in which there is a noticeable sound change within the same syllable. (A single or simple vowel is known as a monophthong.) The process of moving from one vowel sound to another is called gliding, which is why another name for a diphthong is a gliding vowel but they are also known as compound vowels, complex vowels, or moving vowels.

The sound change that turns a single vowel into a diphthong is called diphthongization. Diphthongs are sometimes referred to as "long vowels" but this is misleading. While vowel sounds do change in a diphthong, they do not necessarily take more time to say than a monophthong.

Diphthongs in American English

How many diphthongs are there in the English language? It depends on which expert you ask. Some sources cite eight, others as many as 10. Even syllables containing a single vowel can contain a diphthong. The rule of thumb is: If the sound moves, it’s a diphthong; if it's static, it’s a monophthong. Each of the following diphthongs is represented by its phonetic symbol.

/aɪ/ This diphthong creates sounds similar to "eye" and most often occurs with letter combinations that include /i/, /igh/, and /y. Examples: crime, like, lime

/eɪ/ This diphthong creates sounds similar to “great” and is most often used with letter combinations that include /ey/, /ay/, /ai/ and /a/.

Examples: break, rain, weight

/əʊ/ This diphthong creates sounds similar to “boat” and most often occurs with letter combinations that include /ow/, /oa/ and /o/. Examples: slow, moan, though

/aʊ/ This diphthong creates sounds similar to “ow!” and most often occurs with letter combinations that include /ou/ and /ow/.

Examples: brown, hound, now

/eə/ This diphthong creates sounds similar to “air” and most often occurs with letter combinations that include /ai/, /a/, and /ea/. Examples: lair, stair, bear

/ɪə/ This diphthong creates sounds similar to “ear” and most often occurs with letter combinations that include /ee/, /ie/ and /ea/. Examples: beer, near, pier

/ɔɪ/ This creates sounds similar to “boy” and most often occurs with letter combinations that include /oy/ and /oi/. Examples: oil, toy, coil

/ʊə/This diphthong creates sounds similar to “sure” and most occurs with letter combinations that include /oo/, /ou/, /u/, and /ue/. Examples: lure, pure, fur

Diphthongs in Dialects

One of the most interesting ways in which diphthongs relate to spoken language is in how they’ve evolved into regional accents and dialects from their languages of origin. In the borough Brooklyn, for example, when someone says, “Let the dog out,” the word dog contains a distinctive “aw” sound so that “the dog” becomes a “dawg.”