Vowels are continuous sounds. The air stream expelled from the lungs acquires a distinct quality, but at no point does it meet any obstruction. Mostly tongue is the crucial factor in creating resonance chambers. It can move from a state of total passivity to the highest point in the mouth close to its roof.
This highly flexible organ is capable of positioning itself to various degrees of height. It is in the nature of vowels that they are voiced. What differentiates them is the way the tongue and the lips shape the channel through which the air passes after leaving the larynx. As we see in the word, ‘least’ tongue is raised, lips are spread and in the word ‘loose’ tongue raised at the back, lips are rounded and in the word ‘last’ tongue is low, lips, lips are somewhat rounded. Vowels that are produced when the tongue is high are called Close Vowels, when the tongue is low are called Open Vowels. When the tongue is pushed forward, they are Front Vowels and when pushed backwards are called Back Vowels. When vowels join with other vowels, they become diphthongs because they glide from one vowel sound to the other as ‘O’ in the word ‘go’ is a diphthong. Peter McCarthy states: