What Are Diphthongs, And How Common Are They?

If you’re studying linguistics, languages or even singing, you’ve probably heard of diphthongs. Here’s an overview of what they are.
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What Are Diphthongs, And How Common Are They?

A diphthong is a sound that’s made up of two distinct vowel sounds, where the pronunciation glides from one sound to the other in the same syllable. This word comes from Greek and literally means “double sound.”

In linguistics, they serve to create a distinction with simple vowels, showing us that they act as one vowel sound even though they look like two.

A Mini Introduction To Diphthongs

We generally describe diphthongs as either rising or falling. This refers to the “height” of the vowel sound that ends the diphthong. Rising diphthongs end with a high vowel, like /ʊ/ in English, which appears in words like pull and foot. Meanwhile, falling diphthongs tend to end with a mid or low vowel, such /ə/ in English, which we pronounce like “uh.”

Height in this sense refers to how close the tongue is to the hard palate when producing the vowel sound. High vowels are made with the tongue almost touching the hard palate, while mid or low vowels are made with the tongue closer to the bottom of the mouth, with the tip behind the teeth.

Diphthongs In English

Exactly how many diphthongs English has depends on which variety we’re talking about. However, it’s safe to say that all varieties of English have at least five diphthongs, all rising, indicated by the following lexical sets: PRICE, FACE, CHOICE, GOAT and MOUTH.

In Standard Southern British English (SSBE) and General American English (GenAm), we can broadly transcribe these diphthongs as follows:

  • PRICE /aɪ/, as in try, night and height
  • FACE /eɪ/ as in say, stain and freight
  • CHOICE /ɔɪ/ as in noise, loiter and boy
  • MOUTH /aʊ/ as in noun, cow and shower
  • GOAT /əʊ/ (/oʊ/ in GenAm) as in slow, toe and mauve

Differences Across English Accents

SSBE (and many other non-rhotic varieties of English) has at least one additional diphthong as the result of the loss of historical /r/ after existing vowels. This is the lexical set NEAR, including words such as clear and beer. It’s also a falling diphthong, as it ends with a mid vowel.

Whether or not the vowels in lexical sets of SQUARE and CURE are diphthongs depends again on the exact variety. In SSBE and Australian English, these have both become simple vowels because they’ve undergone a process called monophthongization. (This is the term for when a diphthong becomes a monophthong — the opposite of a diphthong.)

In these varieties of English, the CURE lexical set has now merged with the THOUGHT set, so that the words shore and sure sound identical.

In Welsh English and South African English, on the other hand, these two sets still have the diphthong forms. So if you’re having trouble distinguishing Australian from South African English, listen for words like fair and heir or pure and ensure, and pay attention to whether they have diphthongs or monophthongs. Then you’ll have your answer: Diphthongs mean it’s South African English, whereas monophthongs mean Australian.

Turning back to true diphthongs, how do we know that diphthongs do constitute one vowel sound and not two? By contrasting them with other vowel sounds. Compare the following pairs:

  • buy vs. bee
  • slayed vs. sled
  • loin vs. lawn
  • brown vs. bran
  • coat vs. cot

In each of these examples, the only difference between the pairs is the vowel sound: The first all have diphthongs and the second have simple vowels, but they mean different things. This tells us that a diphthong counts as one vowel sound (or phoneme) and can change the meaning of the word.

Diphthongs In Other Languages

Perhaps unsurprisingly, English is not unique in its use of diphthongs. Many other languages have diphthongs in their vowel systems, and some of them are even very similar to English. 

Other Germanic languages, such as Dutch, German, Norwegian and Icelandic all have several complicated-seeming diphthongs. Meanwhile, Romance languages such as French, Portuguese and Spanish also have diphthongs, though these tend to be much more straightforward. These latter diphthongs are just combinations of the language’s simple vowels plus /w/ or /j/ sounds.

One diphthong that’s nearly universal — because it can or does exist in almost every human language — is /ai/, as in the word eye. This is a very common diphthong across many of the world’s languages, occurring in Icelandic, Welsh, Finnish, Spanish, Hebrew, Japanese, Mandarin and Indonesian.

The next most common diphthong is /au/, as in the exclamation of pain “ow!” This can be found in German, Irish, Czech, Maltese, Arabic and Vietnamese, among other languages.

So in short, don’t be put off by the long name or complicated explanation: Diphthongs are just vowel sounds like any other!

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Sam Wood
Sam is a vegan travel blogger, freelance writer, social media manager, former English teacher, linguist and occasional guest university lecturer from London, now living in Berlin. He is fascinated with the phonetics of the many varieties of English from all over the world, both from a pedagogical and historical point of view and as well speaks German, Spanish and French plus smatterings of Swedish, Mandarin, Arabic and Japanese. In his free time he enjoys watching Star Trek, doing yoga, baking delicious vegan cakes and performing as a drag queen.
Sam is a vegan travel blogger, freelance writer, social media manager, former English teacher, linguist and occasional guest university lecturer from London, now living in Berlin. He is fascinated with the phonetics of the many varieties of English from all over the world, both from a pedagogical and historical point of view and as well speaks German, Spanish and French plus smatterings of Swedish, Mandarin, Arabic and Japanese. In his free time he enjoys watching Star Trek, doing yoga, baking delicious vegan cakes and performing as a drag queen.
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