Diphthongs explained: what they are and how to teach them
In short
A diphthong is a vowel sound that glides from one mouth position to another within a single syllable, like the oi in coin or the ou in out. In phonics, the two main diphthongs are /oi/, spelled oi or oy, and /ou/, spelled ou or ow.
What is a diphthong?
Say the word boy slowly and watch your mouth in a mirror. Your lips start rounded and your tongue moves forward and up as the sound finishes. That glide is what makes a diphthong. The sound changes shape within one vowel, unlike the steady vowel in bed or cat.
Linguists count long i and long a as diphthongs too, since they also glide. In classrooms, though, the word usually refers to the two sounds that have their own spellings: /oi/ as in coin and boy, and /ou/ as in out and cow. This article uses the classroom meaning.
How are the diphthongs spelled?
| Sound | Spelling | Usual position | Examples |
|---|---|---|---|
| /oi/ | oi | middle | coin, boil, point, soil |
| /oi/ | oy | end | boy, toy, enjoy, joy |
| /ou/ | ou | middle | out, loud, cloud, round |
| /ou/ | ow | end, or before n or l | cow, now, down, owl |
The position split is the same one that governs ai and ay. English avoids ending words in i or u, so oi and ou take the middle and oy and ow take the end. Ow also shows up in the middle before n or l, as in town, brown, growl and howl. That exception is reliable enough to teach.
Why does ow have two sounds?
Ow spells /ou/ in cow and long o in snow. Both are common, and there is no rule that tells a reader which to use. The student has to try one, then the other, and pick the real word. Context in a sentence usually settles it, and with words like bow and row, the sentence is the only way to know.
- Teacher
- This word has ow. Try the first sound.
- Student
- /s/ /n/ /ou/. Snou?
- Teacher
- Is that a word?
- Student
- No. Snow, with long o.
- Teacher
- Right. Ow can say /ou/ or long o. Try one, then the other.
How are diphthongs different from vowel teams?
The terms answer different questions. Vowel team describes spelling: two or more letters working together. Diphthong describes sound: a vowel that glides. Oi is both, since it is two letters spelling a gliding sound. Ee is a vowel team but not a diphthong, since the long e sound is steady.
For teaching, the label matters less than the content. Students need to know what sound oi spells and where it goes in a word. Whether you call it a vowel team or a diphthong, keep the term consistent so students are not left wondering if two names mean two things.
How do you teach diphthongs step by step?
- Introduce the sound first. Have students say /oi/ and feel their mouth move.
- Show the two spellings and the position rule: oi in the middle, oy at the end.
- Read a list of words with both spellings, marking each team.
- Sort words into oi and oy columns.
- Spell words from dictation, saying the position rule before writing.
- Read a passage that includes words like coin, boy, enjoy and point.
- Repeat the routine with ou and ow, then add the long o sound of ow.
Teach /oi/ before /ou/. Its spellings are almost perfectly reliable, so students get an easy win first. The /ou/ spellings come with the extra job of ow and the many sounds of ou, as in soup, touch and could, so they need more time.
What about ou in other words?
Ou is one of the least predictable spellings in English. It spells /ou/ in out, long oo in soup, short u in touch and young, the short oo of could, and a schwa in famous. Students should learn the /ou/ sound first, since it is the most common in early books, and meet the others gradually.
In longer words, the -ous ending usually spells a schwa sound followed by /s/, as in famous and nervous. That ending is worth teaching as a chunk in third grade, alongside other suffixes, rather than as part of the diphthong lesson.
When are diphthongs taught?
Usually late in first grade or early in second, after silent e, r-controlled vowels and the main long vowel teams. Students should be comfortable trying more than one sound for a spelling before ow arrives, since that strategy is what makes it manageable.
Plan on about a week for /oi/ and closer to two for /ou/, with the second sound of ow folded in during the second week. Keep both in mixed review for at least a month afterward. These sounds are common in early chapter books, so students who learn them well notice the payoff quickly in their own reading.
What activities help diphthongs stick?
- Mirror check: students watch their lips and jaw move on /oi/ and /ou/, then compare with a steady vowel like the e in bed.
- Position sort: cards with oi, oy, ou and ow words go into middle and end columns.
- Word chains with the new spellings: coin, join, joy, boy, bow, cow, how.
- Silly sentences students write with as many /oi/ or /ou/ words as they can fit.
The silly sentence is the one students remember. 'The loud cow found a brown mouse in the town' gets read, laughed at and reread several times, which is exactly the repetition the spelling needs.