Diphthongs and Long Vowel Sounds: Understanding the Difference
Diphthongs, Long Vowels, and "Bossy R" Sounds: The Vowel Patterns Parents Keep Getting (Understandably) Confused
Have you heard of the word "diphthong" when researching phonics or reading programs?It can also be attached to almost every vowel sound that isn't a plain short vowel. Long A, long I, "oo," "au," genuine gliding sounds like "oi" — all lumped under one banner, all called diphthongs.
So let's sort out, properly, what a diphthong actually is, which sounds are genuinely diphthongs under the NSW English K–10 Syllabus, and where long vowels and "bossy R" sounds fit into the picture based on the NESA syllabus glossary and the phonics research behind the Science of Reading.
Why This Distinction Actually Matters
When children are taught to sound out and spell words, the category a sound belongs to determines which spelling rule applies. Diphthongs like "oi" and "oy" follow a positional rule (oi in the middle of a word, oy at the end). Long vowels follow a completely different rule — the "says its own letter name" pattern, usually taught through magic-e, vowel teams, or a small number of common spelling patterns. Mixing the categories up doesn't just create a labelling error — it teaches the wrong rule for predicting spelling.
Part 1: Long Vowel Sounds
Long vowels are the sounds where a vowel simply "says its own name" — long A sounds like the letter A, long E sounds like the letter E, and so on. Each one can be spelled several different ways.
Long A (as in cake): a_e, ai (rain), ay (play)
Long E (as in tree): ee, ea (beach), y (happy)
Long I (as in bike): i_e, igh (light), y (my)
Long O (as in home): o_e, oa (boat), ow (snow)
Long U (as in cube): u_e, ue (blue), ew (few)
The goal isn't memorising every word individually — it's recognising the pattern, so any new word using the same spelling becomes instantly readable.
Part 2: "Bossy R" Sounds
When R follows a vowel, it takes over the sound completely. Car, her, and bird don't sound anything like their individual letters would suggest — this is exactly why "bossy R" needs to be taught as its own unit rather than sounded out letter by letter.
AR (as in car): car, star, farm
OR (as in fork): or (fork), oor (door), ore (store)
ER / IR / UR (as in her): er (her), ir (bird), ur (turn)
AIR (as in chair): air (chair), are (share), ear (bear)
EAR (as in hear): ear (hear), eer (deer), ere (here)
Worth noting: "ear" genuinely makes two different sounds depending on the word — compare "bear" (AIR sound) to "hear" (EAR sound). That's not an inconsistency in this guide; it's a real feature of English spelling worth pointing out to your child directly.
Part 3: What a Diphthong Actually Is
Here's the part most phonics content gets wrong, so let's be precise.
A diphthong is a vowel sound that glides between two different vowel positions within the same syllable. Say "toy" slowly and pay attention to your mouth: it starts in an "aw" shape and moves to an "ee" shape. That physical glide is the diphthong. Compare that to a short vowel like the "e" in "bed" — your mouth holds one steady position for the whole sound. No glide, no diphthong.
| Category | Sound Behavior | Alphabet Rule | Examples |
|---|---|---|---|
| Long Vowel | Glides phonetically, but represents a single letter's alphabet name | "Says its own name" (A, E, I, O, U) | cake, tree, bite, boat |
| Diphthong | Glides phonetically and creates a brand-new sound | Does NOT say an alphabet letter name | coin, toy, house, cow |
The two true diphthongs
The NSW English K–10 Syllabus (2022) glossary defines diphthong as "a type of vowel phoneme, also known as a glide vowel, formed by combining 2 vowel sounds within a single syllable" — and gives exactly two examples: /oi/ in "join," and /ou/ in "proud." These are the two diphthong groups explicitly taught in NSW classrooms:
OI / OY (as in coin, toy): oi is used in the middle of a word, oy is used at the end.
OW / OU (as in cow, out)
Wait — don't long vowels glide too?
Yes, genuinely and this is the interesting part: long A, long I, and long O do involve a slight glide phonetically (linguists classify /eɪ/, /aɪ/, and /oʊ/ as diphthongs in the strict phonetic sense). If you say "my" slowly, your mouth does move slightly from an open position toward a closed one.
So why aren't they taught as diphthongs? Because oi/oy and ow/ou create entirely new gliding sounds — sounds that don't correspond to any single letter's name — and because of that, they need their own spelling category and their own positional spelling rule. Long vowels, by contrast, are grouped by the "says its own letter name" rule: long O sounds like the letter O, long I sounds like the letter I. That's a different, and for spelling purposes more useful, way of organising these sounds for early readers — even though the phonetic glide is real in both cases.
A simple test you can do with your child: if your mouth glides between two shapes as you say a sound, and that sound doesn't match any letter's name, it's a diphthong. If it glides but still sounds like a letter saying its own name, it's a long vowel.
Other vowel sounds often mistaken for diphthongs
These two sounds get grouped in with diphthongs constantly online, but they don't actually glide — the mouth holds one steady position for the whole sound, making them single, steady vowel sounds (sometimes called monophthongs):
OO (as in moon): oo, ue (blue), ew (chew)
OO (as in book): oo, u (put), ould (could)
AU / AW (as in paw): au (sauce), aw (paw), al (walk)
Say "paw" slowly and you'll notice your mouth doesn't move at all partway through — one steady shape, held for the length of the sound. That's the clearest evidence these aren't diphthongs, regardless of how many phonics worksheets group them that way.
Quick recap — the whole distinction in one test: If your mouth glides between two shapes as you say a sound, and that sound doesn't match any letter's name, it's a diphthong (oi/oy, ow/ou). If it glides but still sounds like a letter saying its own name, it's a long vowel. If your mouth doesn't move at all, it's a steady vowel sound like oo or au/aw.
If a linguist asks you: It's a diphthong. If a primary teacher or reading syllabus asks you: It's a long vowel.
The Study and Learning Treasures
Diphthongs, long vowels, and "bossy R" sounds all get tangled together in generic phonics content because the two-letter spelling patterns look similar on the page. But the sounds behave completely differently in your mouth, and — more importantly for your child's spelling — they follow completely different rules. Teaching the real distinction, rather than a simplified (and sometimes simply incorrect) version, gives your child a much more reliable system for reading and spelling new words they haven't seen before.
Free Download: The Complete Vowel Sounds Guide
Learning and consolidation of these principles plus practice activities with a full answer key are included in a free printable PDF guide you can use at home or in the classroom.
Download the Complete Vowel Sounds Guide (PDF)
It covers all three sound groups from this post in detail, with grapheme tables, example words, and interactive sorting activities for long vowels, bossy R sounds, and true diphthongs.