Diphthongs: oi, oy, ou, and ow Word Lists

A diphthong is one vowel sound that starts in one mouth position and glides toward another. Readsters uses coin and loud as the examples. Cow is a diphthong. The vowel in cow glides. The short a in cat does not. That steady sound is a monophthong: one vowel position, one pure sound.

In reading lessons, the two diphthongs taught as their own set are /oy/ and /ow/. Readsters names those two, with spellings oy as in toy, oi as in void, ow as in cow, and ou as in cloud. Linguists also hear a glide in long a and long i. Reading teachers usually keep cake and bike in the long vowel sounds lessons. This page stays with /oy/ and /ow/.

The spellings are vowel teams, and they are digraphs, because a digraph is two letters. Readsters separates the terms that way: digraphs are letters, diphthongs are sounds. Reading Universe calls a diphthong a vowel team that combines two vowel sounds in one syllable. The vowel teams guide covers the teams that do not glide, such as ai and ee. The digraphs guide covers sh and ch.

oi and oy: the /oy/ sound

Both spellings say /oy/, as in coin and boy. The position is the spelling choice.

oi before a consonant, inside the word: coin, join, soil, point, noise, voice, boil, spoil, foil, moist

oy at the end of a syllable: boy, toy, joy, soy, enjoy, annoy, royal, oyster

Royal and oyster use oy at the end of the first syllable, not at the end of the whole word. That is the pattern to teach: end of the syllable, not only end of the word.

ou and ow: the /ow/ sound

ou before a consonant: house, mouse, cloud, mouth, round, sound, found, count, shout, south

ow at the end, or before n or l: cow, how, now, down, town, brown, clown, owl, growl, flower

Cow is the keyword. If a student asks whether cow is a diphthong, the answer is yes.

Keep the long-o ow words out of this list. Snow, grow, show, own, and know say long o. They belong with the vowel-team lesson, not with cow.

Ten words spelled with au

Caught, haunt, sauce, launch, fault, pause, cause, August, author, applaud.

Those ten words use the spelling au. In most US speech the vowel is /aw/, the same sound as in saw, paw, draw, claw, and straw. It is not the glide in cow. Readsters lists au and aw among vowel spellings that are digraphs, and names /oy/ and /ow/ as the diphthongs reading lessons usually teach. Teach au and aw as the /aw/ sound. Do not tell students that caught glides the way cow does.

How to teach the four spellings

Teach one sound at a time.

• Start with /oy/. Show oi in coin and oy in boy. Sort a handful of each by position: before a consonant, or at the end of the syllable.
• Read the column, then dictate four words from one spelling. Mixing oi and oy on the first dictation hides the position pattern.
• Next day, teach /ow/ the same way: ou in cloud, ow in cow. Put snow in a separate pocket so it is not sorted with cow.
• Add au and aw only after /oy/ and /ow/ are readable. Use saw and caught as the keywords, and say the sound /aw/.
• Read a sentence that uses one spelling. Then a sentence that uses the sound with both spellings.

For kindergarten through 3rd grade, the What Works Clearinghouse practice guide puts decoding, word-part work, and writing on the same plan as daily connected text, in support of accuracy, fluency, and comprehension.

One honest limit: the position pattern is a strong hint, not a guarantee. Owl and brown use ow inside the word. Students still have to meet those words, not force every ow to the end of the line.

Sentences after the sort

A sort is not the last step. Students need lines they can read with only the spellings already taught. The K-2 Decodable Books: 100 Phonics Mini-Books + Wall Cards need a printer, paper, scissors, and a stapler. Delivery is an instant download after checkout: one ZIP with a Start Here guide.