Vowels are voiced sounds produced with a stricture of open approximation without any obstruction, either partial or complete, in the passage of air. Such a definition can help us to distinguish vowels from consonants (in which there is a partial or complete obstruction in the flow of air) but cannot help us to distinguish one vowel from another. For this, we have to set up criteria for the description and classification of vowels. All vowels are normally voiced.

We have to understand two types of differences found in vowels – vowel length and vowel quality. Consider the English words hit and heat. The vowel sounds in these words differ only in their relative length. In heat, the vowel sound is longer than in hit. The second difference to consider is the quality of vowel as is exemplified in the words pool and peel. The vowel in pool is a long, back, high vowel and in peel, it is a long, front, high vowel. The difference in the length of the vowel alone would not give us a complete description of all vowels. It is length together with the difference in the quality of vowels which will help us to distinguish one vowel from another.

In the production of vowels, the air-passage is open for the air to flow freely, without any obstruction. We have to determine the differences in their quality with reference to the modifications that take place in the shape and size of the passages through which the airstream passes – i.e., the pharyngeal cavity, the nasal cavity and the mouth.