Diphthongs

What is a diphthong?

A diphthong (say DIF-thong) is a vowel sound that glides. Your mouth starts in one position and slides to another, all inside one syllable.

Say coin slowly and feel your mouth: it starts round (/o/) and slides toward a smile (/ee/). That moving sound is a diphthong. The word comes from Greek and means "two sounds".

Compare this with the vowel in bed. Your mouth stays still. That is a single, pure vowel.

The main diphthongs for young readers

These are the two diphthongs most phonics programs teach by name. But several long vowels in English are also diphthongs when you listen closely: the long i in bike and the long a in cake both glide.

oi or oy?

Both spell the same gliding sound. The position in the word usually tells you which one to use.

• oi comes in the middle of a word: coin, boil, point, noise, join
• oy comes at the end of a word or syllable: boy, toy, enjoy, joy, royal

ou or ow?

These follow a similar rule.

• ou usually comes in the middle: cloud, house, mouth, round, loud
• ow usually comes at the end or before a final l or n: cow, now, how, owl, brown, town

Watch out: ow has two jobs

The letters ow can make two different sounds:

• the diphthong /ou/: cow, now, crowd
• long o: snow, grow, bowl, show

If one does not make sense, try the other. The /bou/l of soup sounds wrong, so try long o: bowl. A few words even change meaning: a bow on a gift (long o) and take a bow (/ou/).

Diphthongs and vowel teams

A vowel team is any group of letters that spells one vowel sound. Some vowel teams spell diphthongs (oi, oy, ou, ow) and some spell steady long vowels (ee, ai, oa). Learning which is which helps with both reading and spelling.

Reading strategy

1. Spot the vowel team.
2. Ask: "Is this oi/oy or ou/ow?"
3. Say the gliding sound.
4. Blend the whole word, then check it makes sense.

Spelling strategy

When you hear /oi/ at the end of a word, write oy. In the middle, write oi. When you hear /ou/ at the end, write ow. In the middle, ou is the best first guess, though ow sometimes appears before n or l (down, owl).

Worked examples

Example 1: Find the diphthong. The boy found a coin. → oy, ou, oi.

Example 2: oi or oy? destr__, p__nt → destroy, point.

Example 3: Which sound does ow make in grow? → Long o.

Common mistakes

• boi ✗ → boy ✓ (end of word uses oy)
• clowd ✗ → cloud ✓
• Reading snow as /snou/: try the long o sound when /ou/ doesn’t make sense.

Try it yourself

1. Underline the diphthongs: The loud cow ate soil by the house.
2. Fill in oi or oy: enj__, b__l, c__n, t__.
3. Fill in ou or ow: cl__d, h__, m__se, cr__n.
4. Which word has a long o: clown or blown?

Answers: 1. ou (loud), ow (cow), oi (soil), ou (house). 2. enjoy, boil, coin, toy. 3. cloud, how, mouse, crown. 4. blown.

Tip for families

Make two lists on the fridge, "oi/oy" and "ou/ow". Each time someone spots a word with one of these sounds in a book, on a sign or on a cereal box, add it to the list. See which list grows longer by the weekend.

Quick recap

• A diphthong is a vowel sound that glides inside one syllable.
• oi and oy spell /oi/: oi in the middle, oy at the end.
• ou and ow spell /ou/: ou in the middle, ow at the end.
• ow can also say long o, so try both sounds.