A diphthong is a complex vowel sound that begins at one phonetic quality and slides toward another within the same syllable. English contains eight diphthongs, each created by combining two vowel sounds into a single, gliding articulation.
Diphthongs differ from monophthongs (single vowel sounds) because they involve movement of the tongue and lips. This glide creates a distinct sound that functions as one phoneme, or unit of sound, in English. For example, the word "boat" contains the diphthong /oʊ/, which starts near the /o/ sound and moves toward /ɪ/.
| Diphthong | Example Word | Phonetic Symbol |
|---|---|---|
| /aɪ/ | time | /taɪm/ |
| /aʊ/ | out | /aʊt/ |
| /eɪ/ | day | /deɪ/ |
| /ɔɪ/ | boy | /bɔɪ/ |
| /oʊ/ | go | /ɡoʊ/ |
| /ɪə/ | near | /nɪər/ |
| /eə/ | square | /skwɛər/ |
| /ʊə/ | cure | /kjʊər/ |
Many learners confuse diphthongs with digraphs, which are two letters representing one sound (like "ea" in "eat"). While both involve combinations, diphthongs are about sound production, not spelling. Another mistake is treating diphthongs as two separate vowels, but they function as single units in English grammar.
One non-obvious fact: The diphthong /ɪə/ appears frequently in American English but is rare in other dialects. For example, Americans pronounce "idea" as /aɪdɪər/, while British speakers often use /aɪdiːə/.