Diphthongs: oi, oy /ɔɪ/ and ou, ow /aʊ/
Short answer: A diphthong is a vowel that glides from one sound to another in a single syllable. The teams oi and oy say /ɔɪ/ (coin, boy), and ou and ow often say /aʊ/ (house, cow). Start with the first sound and move your mouth smoothly to the second. Oi is used in the middle of a word and oy at the end.
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At a glance
| Letters | Sound | Examples |
|---|---|---|
| oi | /ɔɪ/ | coin, oil, point |
| oy | /ɔɪ/ | boy, toy, enjoy |
| ou | /aʊ/ | house, cloud, out |
| ow | /aʊ/ | cow, now, down |
How to make the sound
- Start with the first part of the sound: /ɔ/ or /a/.
- Glide to the second part: /ɪ/ or /ʊ/.
- Keep it as one smooth syllable.
- Use oy at the end of a word (boy) and oi in the middle (coin).
Examples in sentences
- boy
oy = /ɔɪ/: /bɔɪ/. - coin
oi = /ɔɪ/: /kɔɪn/. - toy
oy = /ɔɪ/: /tɔɪ/. - cow
ow = /aʊ/: /kaʊ/. - house
ou = /aʊ/: /haʊs/. - cloud
ou = /aʊ/: /klaʊd/.
Common problems
- Making two syllables: boy is one syllable, not bo-y.
- Stopping before the glide: Move to the second sound: cow, not cah.
- Reading ow only as /oʊ/: ow can be /aʊ/ (cow) or /oʊ/ (snow). Check the word.
For Vietnamese speakers
- Vietnamese has similar vowel combinations, so use them as a starting point.
- Keep the vowel moving; do not stop on the first sound.
Check yourself
- How do you say “oy” in “boy”?
Show the answer
/ɔɪ/ A glide.
- How do you say “ou” in “house”?
Show the answer
/aʊ/ A glide.
- How many syllables are in “boy”?
Show the answer
One. Not two.
Questions
What is a diphthong?
A vowel sound that glides in one syllable.
When do I use oi and oy?
oi in the middle of a word and oy at the end.
Is ou always /aʊ/?
No: ou can also be /ʌ/ (country), /u/ (soup) and other sounds, but /aʊ/ is the most common.
More lessons in this part
Part of the 👄 Pronunciation tool: Phonics & Word Decoding. See also the English pronunciation guide. Đọc bằng tiếng Việt.
Written for English learners by the English for Fun team. How we write our lessons · Last updated 2026-11-05.
Reviewed by Chinh Q. Hoang, PhD, founder, English for Fun · 2026-10-30