A diphthong is one vowel sound that moves from one mouth position to another. You hear one syllable, not two separate vowel sounds. This glide makes diphthongs different from simple vowels, which stay more steady. English accents do not always use the same diphthongs in the same way.

A simple vowel stays in one main position while you say it. A diphthong changes during the sound, so the tongue or lips move. This movement is small but clear in careful speech. In fast speech, the glide may sound weaker, but it is still one vowel sound.

Word	Notation	Description
see	/iː/	This is a simple vowel because the sound stays steady.
day	/eɪ/	This is a diphthong because the sound glides from one position to another.
go	/oʊ/	This is a diphthong in many accents because the sound moves during the vowel.
my	/aɪ/	This is a diphthong because the sound begins open and ends higher.

Several English diphthongs are common in everyday words. The most common group taught to learners is /eɪ/, /aɪ/, /ɔɪ/, /aʊ/, and /oʊ/. Some accents also use other gliding vowels, but these five are a clear starting set. The same sound can appear in many different spellings.

Word	Notation	Description
say	/eɪ/	This word has the diphthong /eɪ/ with a glide toward a higher front position.
time	/aɪ/	This word has the diphthong /aɪ/ with a glide from open to high front.
boy	/ɔɪ/	This word has the diphthong /ɔɪ/ with a glide toward /ɪ/.
house	/aʊ/	This word has the diphthong /aʊ/ with a glide toward a higher back position.
home	/oʊ/	This word has the diphthong /oʊ/ in many accents with a glide toward /ʊ/.

English spelling does not show diphthongs in one fixed way. One diphthong can match different letter groups, and one letter group can sound different in different words. Learners need to connect sound and word together. Pronunciation is more reliable than spelling alone.

Word or Phrase	Definition
ay and ai	These spellings often represent /eɪ/ in words such as day and rain.
i_e and y	These spellings often represent /aɪ/ in words such as time and my.
oy and oi	These spellings often represent /ɔɪ/ in words such as boy and coin.
ou and ow	These spellings often represent /aʊ/ in words such as house and now.
oa and o_e	These spellings often represent /oʊ/ in many accents in words such as boat and home.

Diphthongs can sound different in slow speech and fast speech. In careful speech, the glide is often more clear. In connected speech, the movement may be shorter, especially in unstressed parts. Different accents may also start or end the glide in a slightly different place.

You can now identify diphthongs as gliding vowel sounds in one syllable. You can tell them apart from simple vowels by listening for movement in the sound. You can match common diphthongs to many everyday words and common spelling patterns. You can also notice that stress, speaking speed, and accent can change how a diphthong sounds.