What Are Diphthongs?
10/23/2026
By Julie Cunningham, MA, CCC-SLP, licensed speech-language pathologist
Short answer: a diphthong is a vowel made of two vowel sounds in one syllable. Your mouth starts in one vowel position and glides to another, so you should feel your jaw, lips or tongue move as you say it. American English has five diphthongs: AY /eɪ/ as in day, AI /aɪ/ as in time, OY /ɔɪ/ as in boy, OH /oʊ/ as in no, and OW /aʊ/ as in how.
Diphthongs are one of the first things I listen for with a new student. When a diphthong loses its movement, late can sound like let. When it starts in the wrong place, mine can sound like main. The good news is that once you know what to feel for, diphthongs are some of the easiest sounds to fix.
What Is a Diphthong?
The word comes from Greek: di means two, and phthong means sound. A diphthong has a starting position and an ending position, and your mouth moves from one to the other. For example, in AY /eɪ/ as in late, your mouth starts open with your tongue forward, then your jaw closes and your tongue arches up toward the roof of your mouth.
The two sounds blend into one vowel, so a diphthong is still one syllable. Day has one syllable, not two.
A vowel that stays in one position is called a monophthong (mono means one), like the AH in father or the UH in butter. If your mouth doesn't move during a diphthong, you're saying it like a monophthong, and it won't sound American.
You may also see diphthongs written with a tie bar, like /e͡ɪ/ on my IPA vowels page. It means the same thing: two sounds that work as one vowel.
The 5 American English Diphthongs
Here are the five diphthongs I teach, with example words and a word pair to test your ear. Each sound has its own free video lesson with close-ups of my mouth.
| Diphthong | IPA | Example words | Can you hear the difference? |
|---|---|---|---|
| AY | /eɪ/ | day, late, payment, Monday | late vs. let |
| AI | /aɪ/ | time, try, vitamin, outside | main vs. mine |
| OY | /ɔɪ/ | boy, voice, point, employer | tie vs. toy |
| OH | /oʊ/ | no, phone, close, hotel | tone vs. town |
| OW | /aʊ/ | how, cloud, shout, shower | load vs. loud |
AY /eɪ/ as in day
Start with your mouth open, your tongue forward and the tip of your tongue low, behind your bottom front teeth. Then close your jaw and arch your tongue up toward the roof of your mouth. Practice words: tray, birthday, payment, away, freeway. Watch the AY lesson.
AI /aɪ/ as in time
Start with your jaw dropped low and your tongue low, with the tip touching the back of your bottom front teeth. Then close your jaw and arch your tongue up. AI starts much lower than AY, so the jaw drop is what keeps mine from sounding like main. Practice words: try, gigantic, vitamin, triathlon, alibi. Watch the AI lesson.
OY /ɔɪ/ as in boy
Start with your lips rounded and your tongue pulled back. Then relax your lips and move your tongue forward and up toward the roof of your mouth. Practice words: toy, voice, point, oil, employer, paranoid. Watch the OY lesson.
OH /oʊ/ as in no
Start with your jaw open and your lips relaxed or slightly rounded. Then close your jaw and round your lips. Two things change here, your jaw and your lips, so watch both in a mirror. Practice words: phone, close, knowing, obey, tiptoe, hotel. Watch the OH lesson.
OW /aʊ/ as in how
Start with the corners of your lips pulled back and your tongue forward, with the tip down behind your bottom front teeth. Then close your jaw a little and round your lips. There's a lot of jaw and lip movement in this one. Practice words: cloud, shout, owl, midtown, shower. Watch the OW lesson.
Are There 5 Diphthongs or 8?
If you learned British English, you may have been taught 8 diphthongs. British textbooks list the same five sounds (with OH written /əʊ/), plus three that glide toward a schwa, in words like near, square and tour. American English keeps the R in those words, so they're R-colored vowels instead: EAR /ɪɹ/, AIR /ɛɹ/ and OOR /ʊɹ/.
Some American textbooks write AY and OH as single vowels, /e/ and /o/, and list only three diphthongs. For speaking, that doesn't change anything. All five need movement to sound American.
How to Spot a Diphthong in Spelling
American English spelling won't tell you much. One diphthong can be spelled many ways, and the same letters can spell different sounds:
- AY /eɪ/: day, late, rain, great, eight, they
- AI /aɪ/: time, my, high, pie, buy, eye
- OY /ɔɪ/: boy, coin, voice
- OH /oʊ/: no, phone, boat, snow, though
- OW /aʊ/: how, out, cloud, town
Notice that ow is OH in snow but OW in town. And a single letter can be a diphthong, like the a in late, the i in time and the o in no. So listen and feel for movement instead of trusting the letters. Phonics classes sometimes call letter pairs like oi and ou "diphthongs," but a diphthong is a sound, not a spelling.
Common Diphthong Mistakes
- No movement. If your first language has pure E and O vowels, like Spanish, Italian or Japanese, you may hold one position for AY and OH. Then AY sounds short and flat, and listeners may hear let when you mean late. If late and let sound alike to you, try my lesson on AY in bait vs. EH in bet.
- Saying only the first half. Many Mandarin speakers say only the first part of AY, sometimes with the jaw too open, so mate sounds like met.
- Starting in the wrong place. AY and AI end in a similar place but start in different places. If your jaw doesn't drop low enough for AI, mine can sound like main. The same thing happens with OH and OW, in tone and town.
- Full movement in every syllable. In a stressed syllable, the pitch of your voice glides up and then down on a diphthong, as in day. In an unstressed syllable, native speakers simplify it: lower in pitch, flatter, shorter, and with less jaw movement, like the AY in always or the OH in meadow. If every diphthong gets full movement, your rhythm can sound slow and over-careful. Learn more about stressed and unstressed syllables.
How to Practice Diphthongs
- Slow down and exaggerate. Hold the sound out, like OOOOYYYY, until you can feel both parts. Then speed back up.
- Use a mirror. You should see your jaw or lips move. If nothing moves, you're saying a monophthong.
- Practice in words, then sentences. Start with the words in the table above, then use them in sentences, like "Today is my birthday" or "I'm talking on the phone."
- Train your ear. Take my free 2-minute American English listening quiz to find the sound pairs you miss.
- Follow a course. My free course, English Pro Lite, has 45 video lessons on vowels, consonants, stress and rhythm. In English Pro, every American diphthong gets its own guided practice in the vowels course, and you can send me your recordings for feedback.
Common Questions
What is a diphthong in simple words?
It's a vowel that slides from one sound to another inside one syllable, like the OY in boy. Your mouth moves while you say it.
What are some examples of diphthongs?
Ten common words with diphthongs: day, late, time, my, boy, voice, no, phone, how and out.
How many diphthongs are in American English?
I teach five: AY /eɪ/, AI /aɪ/, OY /ɔɪ/, OH /oʊ/ and OW /aʊ/. British English is usually taught with eight, because it adds three that glide toward a schwa, in words like near, square and tour. American English uses R-colored vowels in those words.
How do you identify a diphthong?
Say the word slowly in front of a mirror. If your jaw, lips or tongue move during the vowel, it's a diphthong. If they stay in one place, it's a monophthong. Don't rely on spelling: ow is OH in snow and OW in town.
What is a monophthong?
A monophthong is a vowel with one position, like the AH in father or the UH in butter. Your mouth stays still while you say it.
How do you pronounce "diphthong"?
Dictionaries list both DIF-thong and DIP-thong. Many native speakers say DIP-thong, even though the word is spelled with ph.