As for monophthongs the distinguishing quality is produced by the tongue, lips and jaw. The difference is that there is one mouth posture at the beginning of the vowel sound, and another atthe end. The resulting glide between these two tongue and lip positions gives the diphthong its characteristic ´two-sound quality´.
A diphthong is always within a single syllable and perceived as onephoneme, not two; that´s why it occupies only one space in the phonemic chart.
There are two kinds of diphthongs: centring, and closing. When the second element is /J/, the diphthong is calledcentring, because of the glide to a central sound. When the second element is /I/ or /U/ the diphthong is called closing, because the glide is towards closing sounds.
The phonetical characteristics ofdiphthongs are the same as for the vowels: there is no obstruction to the escape o air through the mouth, and they are all voiced, because the vocal cords vibrate in the air flow.
Phonologicallyspeaking, the first element of the diphthong is stronger than the second, so you never reach the full quality of the sound.
In the phonemic chart, the diphthongs respect the same order than thevowels: from front to back.
[pic] as in EAR, BEER, THEATRE, MUSEUM. The starting position is [pic]as in IF or BIT with tongue front and high and lips relaxed but with a smallmovement from spread to open.
[pic]as in TOUR, POOR, SURE, OBSCURE. The starting position is [pic] as in PUT or BOOK, with the lips loosely rounded, becoming neutrally open. This time the smallcontrolled tongue movement goes from the central back position to the mid central position, losing the lip rounding and relaxing your mouth from the tight starting position.
[pic]as in BEAR, PAIR,...