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For a note on how AWE organizes its group of articles on vowels, basically by aspects of sound and of writing, see category:vowels.

A diphthong (derived from the Greek δι-, δισ- (di-, dis-) 'two', 'double' and φθόγγος (phthongos) 'sound', meaning "with two sounds") is a combination of two vowels within the same syllable. It is perhaps best thought of as a vowel glide, where the tongue glides from one position to another.

You can see such a feature very clearly in the -ing participle:

  • In "carrying", which is not a diphthong, the native speaker makes two separate articulations to communicate the two sounds of 'y' and 'i'.
  • Much more common in English are true diphthongs, like the word 'air', where one begins by making an 'a' sound (like the name of the first letter of our alphabet), and then adjusts one's mouth to produce a schwa, a sound much more like the uncertain 'er' of a hesitant speaker, IPA: /ɛə/. In 'noise', for instance, a speaker begins by framing the vowel 'o', as in 'or', and then moves the tongue towards the '-ee-' vowel, as in 'see' - making the latter sound less strong. The result of this is the diphthong '-oi-', IPA: /ɔɪ/. Similarly, but perhaps less obviously, 'hear' and 'here' both contain a glide, or diphthong, moving from the vowel sounding as '-ee-' to the schwa, IPA: /ɪə/.

A diphthong may be written as two different letters, such as the '-oi-' sound in 'boil' or the '-ou-' in 'ouch!'; a single letter (often with a silent '-e-' after the syllable, as in 'made' or 'wide'); or two of the same letters (like the '-ee-' in 'beer').

There are even triphthong glides in English - words like 'power', which starts with an '-a-' sound, before gliding through a '-u-' sound to finish with a schwa, (IPA: /aʊə/) and 'tire', which in some speakers' realisation at least has a similar glide, from '-a-' through '-ee-' to a schwa (IPA: /aɪə/).

The sound of most diphthongs in English is better called a glide - though many books on language call it a diphthong. The reader may see the difference by thinking of the stereotypical pronunciation that an Italian speaker might give to 'ai' - like 'Ah-ee - which is a diphthong; whereas the native English speaker uses a much smoother transition between the starting point and the end of the sound. It is the smoothness of this transition that leads phoneticians to call our version of it a glide. It is also a series of sounds that are very characteristic of the English language. (A vowel produced with no such movement of the tongue is a monophthong.)

Be aware that some writers use the label diphthong for a combination of written letters, which is more properly called a digraph.

AWE has an article on different classifications of dipthong.
See also long vowel - short vowel.