Diphthong
From KneeQuickie
A diphthong is a glide between two vowels, or alternatively, a vowel pronounced with change of height and frontness (and sometimes rounding) from one position to another. "Diphthong" is the most frequently misspelt word in the entire field of linguistics; please note that there are two Hs.
A diphthong may be specified by naming its starting and ending positions; in the IPA and X-Sampa, diphthongs are commonly written just by placing the symbols for the starting and ending vowels next to each other, for example /ai/. Like affricates, diphthongs can also optionally be written with a tie-bar (in X-Sampa, an underscore): /a_i/. (The ending position cannot really be specified precisely, and is often understood as merely specifying the direction that the diphthong moves in from the starting vowel.)
Diphthongs also vary as to whether the starting or ending vowel is more stressed. If the starting vowel is more stressed, the diphthong is called falling; if the ending vowel, rising. Note that these terms have nothing to do with the height of the vowels involved; indeed, falling diphthongs most commonly move from a lower vowel to a higher one, and rising diphthongs from a higher vowel to a lower!
Diphthongs are found in many natlangs. In most dialects of English, for example, the /e/ and /o/ phonemes are in fact pronounced as diphthongs, [eI] or [EI] and [oU] or [@U]. In other languages, such as Spanish, /e/ and /ei/ are considered separate phonemes.
There are no rules for how many diphthongs a language may have. Some languages have none; others have more diphthongs than "pure" vowels.
The term "diphthong" is often loosely applied to a vowel either followed or preceded by a semivowel; for instance, /aj/ instead of /ai/. However, there is a difference, in that the tongue actually touches the palate for /j/, but not for /i/. Some languages even have a phonemic distinction between /ai/ and /aj/.
