being open and the tongue being positioned low in the mouth. Height is defined by the inverse of the F1 value: the higher the frequency of the first formant, the lower (more open) the vowel. In John Elsing's usage, where fronted vowels are distinguished in height by the position of the jaw rather than the tongue, only the terms 'open' and 'close' are used, as 'high' and 'low' refer to the position of the tongue. The International Phonetic Alphabet defines seven degrees of vowel height, but no language is known to distinguish all of them without distinguishing another attribute: close (high) near-close (near-high) close-mid (high-mid) mid (true-mid) open-mid (low-mid) near-open (near-low) open (low) The letters are typically used for either close-mid or true-mid vowels. However, if more precision is required, true-mid vowels may be written with a lowering diacritic . The Kensiu language, spoken in Malaysia and Thailand, is highly unusual in that it contrasts true-mid with close-mid and open-mid vowels, without any difference in other parameters like backness or roundness. It appears that some varieties of German have five vowel heights that contrast independently of length or other parameters. The Bavarian dialect of Amstetten has thirteen long vowels, which can be analyzed as distinguishing five heights (close, close-mid, mid, open-mid and open) each among the front unrounded, front rounded, and back rounded vowels as well as an open central vowel, for a total of five vowel heights: . No other language is known to contrast more than four degrees of vowel height. The parameter of vowel height appears to be the primary cross-linguistic feature of vowels in that all spoken languages that have been researched till now use height as a contrastive feature. No other parameter, even backness or rounding (see below), is used in all languages. Some languages have vertical vowel systems in which at least at a phonemic level, only height is used to distinguish vowels. Backness Vowel backness is named for the position of the tongue during the articulation of a vowel relative to the back of the mouth. As with vowel height, however, it is defined by a formant of the voice, in this case the second, F2, not by the position of the tongue. In front vowels, such as , the frequency of F2 is relatively high, which generally corresponds to a position of the tongue forward in the mouth, whereas in back vowels,