A vowel diagram or vowel chart is a schematic arrangement of the vowels. Depending on the particular language being discussed, it can take the form of a triangle or a quadrilateral. Vertical position on the diagram denotes the vowel closeness, with close vowels at the top of the diagram, and horizontal position denotes the vowel backness, with front vowels at the left of the diagram.[1]
The vowel systems of most languages can be represented by vowel diagrams. Usually there is a pattern of even distribution of marks on the chart, a phenomenon that is known as vowel dispersion. For most languages, the vowel system is triangular. Only 10% of languages, including the English language, have a vowel diagram that is quadrilateral. Such diagrams are termed vowel trapezia or vowel quadrilaterals. German phonologists know these as, respectively, a Vokalviereck and a Vokaltrapez.[1]
The IPA vowel chart comprises the cardinal vowels, and is displayed in the form of a trapezium. By definition, no vowel sound can be plotted outside of the IPA trapezium because its four corners represent the extreme points of articulation. The vowel diagrams of most real languages are not so extreme. In English, for example, high vowels are not as high as the corners of the IPA trapezium, nor are front vowels as front.[1][3]