What Is a Diphthong
The word diphthong comes from the Greek diphthongos which means two tones or two sounds. The dictionary defines a diphthong as
"an unsegmentable, gliding speech sound varying continuously in phonetic quality but held to be a single sound or phoneme and identified by its apparent beginning and ending sound, as the oi- sound of toy or boil."
In English, we have many words with gliding vowels or diphthongs, which means that the words have two vowel sounds in one syllable. Most linguists agree that there are eight diphthongs in the English language (although some argue for as few as three). The most common diphthongs in American English are said to be /ow/, /ou/, /oy/ and /oi/ as in row, round, joy and join. Diphthongs, however, do not always have two written vowels side-by-side in the syllable that makes the word. For instance, five has the diphthong /ai/.
In teaching diphthongs, linguists suggest that we think of the vowel sound moving from one vowel to another, thus "gliding" into the final sound. For instance, if we use the word hound, we hear what sounds like "ha" followed by "ou". The first sound flows into the final sound to create the word.
Another consideration in teaching American English diphthongs is that of dialect. In various parts of the country, words are pronounced with diphthongs that are not heard anywhere else. For instance, in New England, the word hair is pronounced "hay-uh, rather than "hay-er."