The last unit talked about simple vowels, where the tongue position stays pretty constant throughout the duration of the vowel. In addition to simple vowels, many languages include diphthongs, where we move our articulators while producing the vowel. This gives the sound a different a different shape at the end from how it begins. The word diphthong comes from the Greek word for “two sounds”.
There are three major diphthongs in English that have quite a noticeable change in the quality of the vowel sound.
Say these English words out loud: fly, tie, ride, smile. Now make the vowel sound [aɪ] again but hold it at the beginning [aaa]. The first part of the sound is the low front [a], but then the tongue moves up quickly at the end of the sound, ending it [ɪ]. So the [aɪ] sound is a diphthong, and it gets transcribed with two consecutive symbols:[aɪ].
In the words now, loud, brown, the tongue again starts low and front [a], and then it moves high and to the back of the mouth, and the lips get rounded too! The second part of this diphthongs is but the high back rounded [ʊ]. The [aʊ] diphthong is transcribed like this: [aʊ].
The third major diphthong in English occurs in words like toy, boil, coin. It starts with the tongue at the back of the mouth and lips rounded [ɔ], then moves to the front with lips unrounded. It is transcribed like this: [ɔɪ].
Some linguists also consider the vowel sound in cue and few to be a diphthong. In this case, the vowel sound starts with the glide [j] and then moves into the vowel [u].
In addition to these major English diphthongs, speakers of Canadian English also have a tendency to turn the mid-tense vowels into diphthongs.
For example, let’s look at the pair of vowels [e] and [ɛ] from the words gate and get. They’re both mid, front, unrounded vowels, but [e] is tense – it’s made with greater tension in the muscles of the vocal tract than [ɛ]. Canadian English speakers pronounce the lax vowel in get as a simple vowel [ɡɛt], but for the tense vowel, we tend to move the tongue up at the end: [ɡeɪt]. We do it so systematically that it’s very hard for us to hear it, but it’s always there.
We do the analogous thing for the mid-back vowel [o] like in show and toe: at the end of the [o] vowel, the tongue moves up a little bit so we produce the vowel as [oʊ]. Notice that the lips are rounded for both parts of this diphthong.
To sum up, a diphthong is a vowel sound that involves movement of the tongue from one position to another. Nearly all dialects of English include the three major diphthongs [aɪ] , [aʊ] , and [ɔɪ]. These ones are called the major diphthongs because they involve large movements of the tongue.
In Canadian English, speakers also regularly produce diphthongs for the tense vowels, [eɪ] and [oʊ], but not all English dialects do this. Some linguists consider these ones to be minor diphthongs.