Abstract
This research aim at classifying the theoretical and practical principles of vowels. Some of these principles are related to the acoustic features. The acoustic features can be observed in sonority and friction. The contextual criteria are observed in the phonological usages and practical linguistic level. However, the practical criteria can be easily observed in the articulatory features such as narrowing and the obstruction of the air- system.
Cardinal vowels represent the duality of vowels, they represent the theoretical and practical aspects of vowels. Cardinal vowels are divided into: primary, secondary, and additional vowels. All cardinal vowels are defined according to the following criteria: the horizontal and vertical position of the tongue, and the position of the lips (rounded, unrounded, neutral).
This paper discusses as well the relation between the so called diphthongs and semivowels. Diphthongs have been divided into five types.