A vowel diagram or vowel chart is a schematic arrangement of vowels within a phonetic system. Vowels do not differ in place, manner, or voicing in the same way that consonants do. Instead, vowels are distinguished primarily based on their height (vertical position), backness (horizontal position), and roundedness (lip articulation). The degrees of height and backness may be used to describe either tongue articulation or acoustic quality, depending on the method of analysis. The former is the more traditional method, while the latter is the more modern method.
The vowel systems of all languages can be represented by vowel diagrams. Usually, there is a pattern of even distribution of vowel placement on the diagram, a phenomenon that is known as vowel dispersion. Most languages have a vowel system with three extreme points, forming a vowel triangle. Only 10% of languages, including English, have a vowel system with four extremes. Such a diagram is called a vowel quadrilateral or a vowel trapezium.
Alternatively, these shape-based terms are used to distinguish the method of analysis used for vowel diagrams: vowel triangles for diagrams typically based on acoustics, and vowel quadrilaterals for diagrams typically based on articulation. As such, languages which have three extremes may still be plotted within a vowel quadrilateral, and languages which have four extremes may still be represented with a vowel triangle.
The vowel diagram of the International Phonetic Alphabet is based on the cardinal vowel system, displayed in the form of a trapezium. In the diagram, convenient reference points are provided for specifying tongue position. The position of the highest point of the arch of the tongue is considered to be the point of articulation of the vowel.
In the IPA Handbook, the vowels are regarded as fixed articulatory reference points, with precise tongue positioning, while the remaining vowels are defined "so that the differences between each vowel and the next in the series are auditorily equal". As such, the Handbook concedes:
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 articulated lower than in the IPA trapezium, and front vowels are articulated further back. As such, vowel diagrams are often transcribed with broader vowel phonemes rather than narrower vowel phones, omitting the usage of diacritics; the latter may be visually interpreted from a given diagram. This is also the case as vowel diagrams typically intend to represent averages among speakers of a language, rather than definitive placements for all individuals.
Vowels may also be categorized by their perceived tenseness, with lax vowels being positioned more centralized on vowel diagrams than their tense counterparts. The vowel is in the center of the IPA trapezium and is frequently referred to as the neutral vowel, due to its fully lax articulation. In many languages, including English, the vowels and are often considered lax variants of their tense counterparts and , and are placed more centralized in the IPA trapezium. See also vowel reduction and checked and free vowels.
Different vowels vary in pitch. For example, high vowels, such as and , tend to have a higher fundamental frequency than low vowels, such as . Vowels are distinct from one another by their acoustic form or spectral properties. Spectral properties are the speech sound's fundamental frequency and its formants.
According to Peter Ladefoged:
Each vowel in the vowel diagram has a unique first and second formant, or F<sub>1</sub> and F<sub>2</sub>. The frequency of the first formant is associated with the width of the pharyngeal cavity and the position of the tongue on a vertical axis, and ranges from open to close. The frequency of the second formant refers to the length of the oral cavity and the position of the tongue on a horizontal axis. , , are often referred to as point vowels because they represent the most extreme F<sub>1</sub> and F<sub>2</sub> frequencies. has a high F<sub>1</sub> frequency because of the narrow size of the pharynx and the low position of the tongue. The F<sub>2</sub> frequency is higher for because the oral cavity is short and the tongue is fronted in the mouth. The F<sub>2</sub> frequency is low in the production of because the mouth is elongated and the lips are rounded while the pharynx is lowered.
F<sub>1</sub> and F<sub>2</sub> are not the only formants used to measure vowels. The third formant, or F<sub>3</sub>, is also often used for measuring vowel quality, but it is not typically demonstrated on a dual-axis chart as F<sub>1</sub> and F<sub>2</sub> are. The lowering of F<sub>3</sub> is commonly associated with front rounded vowels, such as , and r-colored vowels, such as ; the latter to a greater degree, being a marked characteristic of r-colored vowels.
The official vowel chart of the International Phonetic Alphabet does not include vowel symbols with added diacritics as shown here, and only gives labels for the heights "close", "close-mid", "open-mid", and "open" (shown here in bold).