Vowels, therefore, are defined as those sounds with which the air passes without major obstruction from the vocal folds to the lips.
As was discovered by phonetics researchers such as Daniel Jones in the late 19th and early 20th century, the major determinant of vowels was the position of the tongue in the mouth. It was found that vowels could be identified according to two major variables: which part of the tongue was raised in the making of the sound (front, middle or back, and the degree to which the tongue was raised (high in the mouth towards the hard palate, midway in the mouth or low in the mouth). Daniel Jones developed what is known as the `cardinal vowel diagram' to map these possibilities and to give a frame of reference for the identification of vowel sounds in all languages and in all varieties within a language.
Under the cardinal vowel system, four primary tongue heights are identified - low, low-mid, high-mid and high. The lower the tongue in the mouth, the more open the mouth and the higher in the moth the more closed the mouth. These four positions are therefore identified as open (tongue at lowest position), open-mid (tongue slightly raised), close-mid (tongue further raised) and closed (tongue raised to highest point without obstructing the air flow). Similarly, the system also identifies three primary front-to-back positions for the part of the tongue which is involved (front, central and back). See the diagram below.
Figure 1: Cardinal vowels
A symbol was assigned to for each of the possible intersection points between height and front-to-back position. Thus a symbol was assigned to the sound that is made with front part of the tongue raised in its highest position (Close/Front, cardinal vowel 1), another for the sound that is made with the front part of the tongue in a mid-high position (Close-mid/Font, cardinal vowel position 2) and so on. It is important to bear in mind that that the identification of these cardinal tongue positions was, in theory, made independently of any particular language. That is to say, it is an artificial system. Thus to make the sound associated with position 1 (most closed, most forward position) a phonetician will often have to learn to place their tongue in the required position, since they may not have a high, front vowel in their language or because the highest, most frontward vowel in their language may not, in fact, be the highest, most frontward vowel that is possible. Their highest most frontward vowel, therefore, may be somewhat lower, or somewhat further back than position 1 on the cardinal vowel diagram.
The artificial nature of the vowel positions on the cardinal vowel diagram is obscured by the fact that, in many cases, letters from the Latin alphabet are used to reference particular points. This easily leads to the wrong conclusion that sounds at the cardinal vowel points are necessarily the same as the sounds for which those letters stand. The artificial nature of the cardinal vowel diagram is perhaps more obvious in the context of those symbols which are not taken from the Latin alphabet, symbols such as _.
The points on the cardinal vowel diagram, therefore, are artificial points of reference. In order to identify the sounds of a given dialect or language, the phonetician seeks to map the vowel sounds of that language against those cardinal points. By `map', we mean take each vowel sound and discover which of the cardinal vowels it most closely resembles. The symbol for this cardinal vowel (for example, the [i] of the highest, most front-ward vowel) is then assigned to the sound it most closely resembles. Thus in English phonetics, the symbol [i] is assigned to the most closed (tongue at the highest), most frontward vowel sound in the language - the a sound in heed. This is despite the fact that the a sound in heed, is, in fact, slightly more open (tongue lower) than the cardinal [i] in the cardinal vowel diagram. The symbol [i] is nevertheless assigned because the ee sound in heed is in the most closed, most forward position, of all sounds of English and because it is quite close phonetically to the cardinal [i]. Thus the mapping of the sounds of a particular language with the sounds of the cardinal vowel system is, in a sense, a fudge. When we assign the symbol [i] to the vowel in had we are to a certain degree ignoring the fact that it is not, actually, the most open, most frontward vowel sound that the human voice can, in principle, make. In many instances this slight fudge is not a problem. It is often enough to identify which cardinal vowel sound the vowel in question most closely resembles. If more detailed phonetic descriptions are required, then the International Phonetic Alphabet which supplies the symbols for phonetic transcription does contain the necessary diacritics for indicating such details of forwardness of openness. (For a very helpful map of precise relationship of the English vowels to the cardinal vowel positions see the central column, page 156 of the Crystal readings.
