What to hear a scary secret? More than 63% of kids who struggle to decode words with complex vowel patterns in early grades will continue to struggle with reading fluency and comprehension into middle school.
Why? Because too many reading programs gloss over or rush through advanced phonics patterns like diphthongs—those vowel combinations that change their sound in surprising ways.
They’re not rare exceptions. They’re everywhere in English: in words like coin, loud, boil, house, and now. And if kids don’t master them, they’re left guessing at words, memorizing instead of decoding, and hitting walls as texts get harder.
If you're a teacher, tutor, or parent committed to effective literacy instruction, you can't afford to skip or "sort of mention" diphthongs. You need to teach them explicitly, systematically, and with intention.
This article is your guide to understanding what diphthongs are, why they matter so much in reading development, and how to teach them in ways that actually work.
What Are Diphthongs? A Clear, Teacher-Friendly Definition
When people hear “diphthong,” they sometimes think it’s an intimidating linguistic term best left to experts. But it’s simple:
A diphthong is a single vowel sound that begins with one vowel sound and glides into another within the same syllable.
Instead of one pure vowel sound (like the “a” in cat), you get a complex sound movement (like the “oi” in boil or the “ou” in house).
Examples of common English diphthongs include:
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/oi/ as in coin, boil, toy
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/ou/ as in house, out, shout
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/au/ (or /aw/) as in author, claw, draw
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/ai/ and /ay/ as in rain, play (though these are sometimes classified with other vowel teams, they share diphthong-like gliding properties in speech)
Unlike static long or short vowels, diphthongs move. And that movement is exactly what makes them tricky for early readers—and why they need special attention in your phonics instruction.
Why Diphthongs Matter: The High-Stakes Case for Teaching Them Explicitly
It’s easy to assume that kids will just “pick up” these vowel combinations with enough exposure. After all, they see them in books all the time, right?
But research says otherwise. Studies show that children need explicit, systematic phonics instruction to map sound-spelling relationships accurately. That includes diphthongs.
When kids don’t master diphthongs, they often:
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Guess wildly at unfamiliar words.
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Rely on memorizing whole words, exhausting their working memory.
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Struggle to decode multisyllabic words with diphthong patterns.
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Lose confidence and avoid challenging texts.
Think about it: you can’t read loud, boil, or shout without understanding diphthongs. And they’re not rare, high-level words—they’re basic vocabulary found in early readers.
If you're aiming for Reading intervention that actually closes gaps, or building a strong Homeschool reading plan, you have to address diphthongs directly.
How to Teach Diphthongs: The Science-Backed Approach
So how do you move beyond hoping kids will just “get it”?
You teach diphthongs explicitly and systematically, like any other phonics skill. That means:
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Introduce the sound clearly. Say the diphthong sound slowly, exaggerating the glide. For example, with /oi/, start with “o” and slide to “i.”
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Show the spelling patterns. Teach common ways diphthongs are spelled. For /oi/, show “oi” (as in coin) and “oy” (as in toy).
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Provide Phonics examples in isolation. Practice reading words like boil, coin, soil, toy, joy.
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Move to connected text. Use diphthongs decodable texts that carefully control for these patterns. This ensures kids aren’t overwhelmed with untaught phonics patterns while practicing diphthongs in real reading.
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Include writing and spelling practice. Dictate words and sentences containing the diphthong. Have kids identify the sound and choose the correct spelling.
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Revisit and review. Don’t teach it once and move on. Spiral back regularly so the knowledge sticks.
What’s critical here is that diphthongs aren’t presented as exceptions or “just memorize it” items. They’re taught as patterns kids can decode and spell, giving them the tools to tackle new words with confidence.
Making It Engaging: From Drill to Delight
Teaching diphthongs doesn’t have to be boring. In fact, it shouldn’t be.
Great teachers use storytelling, visuals, and multisensory techniques to bring these patterns to life. For example:
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Use gesture or motion to show the “slide” in a diphthong.
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Create word sort games that let kids compare diphthong patterns.
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Use songs or chants with diphthong-heavy rhymes.
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Incorporate diphthongs decodable texts that tell funny or adventurous stories kids want to read.
That last one is crucial. Controlled text doesn’t have to be dull. With the right decodables, you can keep kids engaged while they practice these essential patterns.
Diphthongs in Reading Intervention and Homeschool Settings
If you’re working in reading intervention, you know that older struggling readers often have gaps with vowel teams and diphthongs. They might breeze through simple CVC words but freeze at boil or house.
That’s why any effective intervention plan includes a diagnostic approach: find where the gaps are, reteach the patterns, and provide lots of supported practice.
For Homeschool reading, diphthongs are equally important. Many homeschool curricula focus on early phonics (letters and sounds, short vowels) but don’t offer a clear plan for advanced vowel patterns. That can leave kids stuck when they hit more complex books.
Building diphthong instruction into your phonics sequence ensures you’re preparing your child for the real complexity of English—not just the easy words.
Practical Strategies and Phonics Examples
If you want to make your instruction effective, focus on these simple but powerful strategies:
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Model the sound explicitly. Don’t assume they hear the glide—show it.
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Use clear Phonics examples that highlight the target pattern. Don’t bury diphthongs in text filled with other untaught patterns.
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Practice reading AND writing. Encoding reinforces decoding.
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Provide cumulative review so they don’t forget it next month.
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Use decodable texts that actually feature diphthongs, not random sight words or guessable patterns.
The Long-Term Payoff: Why Mastering Diphthongs Matters
This isn’t about passing a phonics test. It’s about building true reading independence.
Kids who master diphthongs can decode hundreds of essential English words automatically. That frees their mental energy for comprehension, fluency, and enjoyment.
It also sets them up for success as texts get more complex. Without this skill, reading remains a guessing game—a frustrating struggle that turns kids off books entirely.
Whether you're working in a classroom, running small-group reading intervention, or planning homeschool reading lessons, teaching diphthongs systematically is a gift you give your students for life.
Because in the end, reading isn’t about guessing. It’s about knowing. And when you teach diphthongs well, you give kids one more crucial key to unlock the entire world of words.