Diphthong
A diphthong is a vowel sound that involves a glide from one vowel quality to another within a single syllable. The tongue starts in one position and moves toward another, producing a perceptible change in quality. The word comes from Greek di- (two) + phthongos (sound). Diphthongs contrast with monophthongs (pure vowels), where the tongue position remains relatively stable throughout (Roach, 2009; Ladefoged & Johnson, 2015).
The English Diphthong System
Standard British English (RP) has eight diphthongs, traditionally grouped into three sets based on the direction of the glide:
Closing diphthongs (glide toward a closer vowel)
These move toward /ɪ/ or /ʊ/:
| Diphthong | Example | Starting position | Glide toward |
|---|---|---|---|
| /eɪ/ | day, face, make | mid front | /ɪ/ |
| /aɪ/ | time, price, my | open front | /ɪ/ |
| /ɔɪ/ | boy, choice, join | open-mid back rounded | /ɪ/ |
| /əʊ/ | go, goat, home | mid central | /ʊ/ |
| /aʊ/ | now, mouth, town | open front | /ʊ/ |
Centring diphthongs (glide toward schwa /ə/)
| Diphthong | Example | Starting position | Glide toward |
|---|---|---|---|
| /ɪə/ | near, here, beer | close front | /ə/ |
| /eə/ | square, fair, there | mid front | /ə/ |
| /ʊə/ | tour, cure, poor | close back | /ə/ |
Notes on variation
The centring diphthongs are undergoing change in contemporary British English. /ʊə/ is merging with /ɔː/ for many speakers (poor = pour). /eə/ is increasingly monophthongized to [ɛː]. In General American English, the centring diphthongs are replaced by vowel + /r/ sequences due to rhoticity: near = /nɪr/, square = /skwer/.
General American also differs in /əʊ/, which is typically realized as /oʊ/ (starting further back and more rounded).
Diphthongs vs. Monophthongs
The phonemic status of diphthongs is debated. Some phonologists (e.g., Gimson, 1962) treat them as single phoneme units occupying one vowel slot; others analyze closing diphthongs as vowel + glide sequences (/eɪ/ = /e/ + /j/). For ELT purposes, the standard treatment is as single phonemes — they occupy one syllable nucleus, take one beat, and pattern as units.
Key test: A diphthong occupies one syllable. Boy /bɔɪ/ is one syllable; boa /bəʊ.ə/ has the diphthong /əʊ/ in the first syllable and /ə/ in the second. Two adjacent vowels in separate syllables (a hiatus) are not a diphthong.
Triphthongs
When a closing diphthong is followed by /ə/, a triphthong results — a glide through three vowel qualities in one syllable: fire /faɪə/, power /paʊə/, layer /leɪə/, loyal /lɔɪəl/, lower /ləʊə/. In practice, triphthongs are heavily compressed in connected speech and often reduced to diphthongs or even monophthongs: fire → [fɑː], power → [pɑː] in rapid speech (Roach, 2009).
Why It Matters for ELT
L1 transfer and monophthongization. Many languages have fewer diphthongs than English, and learners tend to monophthongize — replacing a glide with a pure vowel. Spanish speakers may produce /eɪ/ as [e], making late sound like let. Vietnamese speakers may reduce /əʊ/ to [o]. Arabic speakers often struggle with centring diphthongs entirely. Teaching the glide movement is essential.
Spelling-sound mismatch. English diphthongs have notoriously inconsistent spelling: /eɪ/ can be spelled a-e (make), ai (rain), ay (day), ey (they), ei (vein), ea (great). This makes diphthongs a key area where phonemic transcription helps learners see through spelling to the actual sound system.
Vowel length and rhythm. Diphthongs are inherently long — they take time to complete the glide. This affects rhythm and sentence stress. In unstressed syllables, diphthongs may be shortened or reduced, contributing to the compression patterns of connected speech.
The /ɪ/-/iː/ contrast and diphthongs. Many learners confuse the close monophthongs with the closing diphthongs: /ɪ/ (bit) vs. /ɪə/ (beer), /ʊ/ (full) vs. /ʊə/ (tour). Minimal pair work helps, but teachers need to emphasize the glide movement, not just the length difference.
Teaching technique. Use hand gestures showing the glide direction (hand moving up for closing diphthongs, toward center for centring diphthongs). The physicality helps learners produce the movement rather than a static vowel. Rubber-banding (stretching the diphthong exaggeratedly, then returning to natural speed) is effective for production practice.
Key References
- Roach, P. (2009). English Phonetics and Phonology (4th ed.). Cambridge University Press. — Chapter 8 on diphthongs and triphthongs.
- Kelly, G. (2000). How to Teach Pronunciation. Longman. — Chapter 2 on vowels and diphthongs for teaching.
- Celce-Murcia, M., Brinton, D., & Goodwin, J. (2010). Teaching Pronunciation (2nd ed.). Cambridge University Press. — Chapter 5 on the vowel system including diphthongs.
- Gimson, A.C. (1962). An Introduction to the Pronunciation of English. Edward Arnold. — Classical description of the RP diphthong system.
- Ladefoged, P. & Johnson, K. (2015). A Course in Phonetics (7th ed.). Cengage. — Acoustic and articulatory analysis of diphthongs.
- Cruttenden, A. (2014). Gimson's Pronunciation of English (8th ed.). Routledge. — Updated RP diphthong descriptions reflecting ongoing change.