Diphthong Words
A diphthong is a vowel sound that glides: the mouth starts in one position and moves to another inside a single syllable. Say "oil" slowly and you can feel it. The two diphthongs students learn to spell first are /oi/ as in coin and boy, and /ou/ as in cloud and cow.
Each diphthong has two spellings, and position in the word does most of the deciding. OI works at the start or in the middle of a word (oil, boil, coin, point) while OY takes the end (boy, toy, joy, enjoy). OU works at the beginning and middle (out, loud, found) while OW usually handles the end (cow, now) and appears before endings like n, d, and er (brown, crowd, flower).
The lists below come straight from our Grade 3 spelling curriculum: OI/OY words and OU/OW words, each printable as worksheets and ready for free online practice.
The rule
A diphthong is a gliding vowel sound: /oi/ as in coin and boy, /ou/ as in cloud and cow. Position usually decides the spelling: oi and ou at the start or middle of a word (coin, loud), oy and ow at the end (boy, cow).
Diphthong Words by grade, from our curriculum
These are the exact lists our K-6 spelling curriculum teaches. Every list links to free online practice and printable worksheets - no account needed.
Diphthong OI/OY Words
Grade 3- oil
- boil
- coin
- point
- oink
- boy
- toy
- joy
- enjoy
- employ
Diphthong OU/OW Words
Grade 3- out
- loud
- found
- sound
- cloud
- cow
- now
- brown
- flower
- crowd
How the pattern works
Unlike a long or short vowel, a diphthong is a moving sound: in coin the vowel starts like "aw" and glides toward "ee". That glide is why these are taught as their own group rather than as long or short vowels.
For /oi/ the position rule is nearly airtight: apart from a few borrowed words like koi, English words do not end in oi, so the end-of-word spelling is oy (boy, joy, enjoy, employ) and the start-or-middle spelling is oi (oil, boil, coin, point).
For /ou/ the rule is a strong guide but not perfect: ou starts and fills the middle of words (out, loud, found, sound), and ow takes the end (cow, now). Before a final consonant both appear: cloud spells it ou while crowd spells it ow, so a few words need word-level memory.
Common mistakes to watch for
- Ending a word in oi ("boi" for boy, "enjoi" for enjoy) - fixed with the position rule: oy does the end-of-word work.
- Swapping ou and ow inside words ("broun" for brown, "clowd" for cloud) - expected, since both spellings are legal there; these few words are learned individually.
- Confusing ow the diphthong (cow, how) with ow the long o team (snow, grow) - same letters, two different sounds, and only the word decides.
- Writing a single vowel for the glide ("fond" for found) when writing quickly - the second half of the glide gets lost.
Example sentences
- coin - He found a shiny coin under the bench.
- boy - The boy waved from the bus window.
- cloud - One gray cloud drifted across the sun.
- brown - She drew a brown horse in her notebook.
- enjoy - We enjoy reading time right after lunch.
Taught in Grade 3 in our curriculum, after the long-vowel teams are secure. OI/OY comes first, then OU/OW, so students learn each position rule with fresh examples.
Frequently asked questions
What is a diphthong?
A vowel sound that glides from one mouth position to another within one syllable: /oi/ in coin, /ou/ in cloud. It is neither a long nor a short vowel - it moves.
When do you use oi vs oy?
Position decides: oi spells the sound at the start or middle of a word (oil, coin, point) and oy spells it at the end (boy, toy, joy, enjoy), because English words almost never end in oi - koi is a rare borrowed exception.
When do you use ou vs ow?
Ou works at the beginning and middle (out, loud, found); ow takes the end of a word (cow, now) and appears before endings like n, d, and er (brown, crowd, flower). The guide is strong but not perfect - cloud and crowd have to be remembered as individual words.
Is the ow in snow a diphthong?
No. In snow and grow, ow is a vowel team spelling long o - a single, steady sound. In cow and how, the same letters spell the gliding diphthong. The spelling is identical; the sound is not.
Turn these lists into practice in one click
Free spelling games, tests, and printables for every list above - built for K-6 classrooms and home practice. No signup, no cost.