[Decimus]

Originally Posted by steven

I will argue with you that あい is not spoken as two distinct syllables. It is one syllable. My point was that while the sound may not gllllllllllieeeeede like so many vowels in the English language, but that they combine to create a vowel outside of the "5 vowels of Japanese" that people like to talk about.

I will also argue that while "ai" might not be a good example of one, dipththongs do exist in Japanese. I want to stress that I did not mean that "あい" is pronounced the same way as English's "eye", but that it is similar.

A diphthong 「二重母音」, by its very definition, must have a glide.

diphthong /'difthan/ n.
A single syllabic nucleus which begins with one vowel quality and changes more or less smoothly to a second quality, as in [ju] and [ai]. Usually one one of the two vocalic elements is more prominent than the others, this other consisting only of a preceding glide (an on-glide, as in [ju]), or a following glide (an off-glide, as in [ai]). The first type is called a crescendo (or rising) diphthong, the second a diminuendo (or falling) diphthong. Diphthongs may be further classified as wide or narrow, as closing or opening, or as backing, fronting or centring. Cf. monophthong, triphthong, tetraphthong.
NOTE the spelling and the pronounciation of this word. It is an error to use the term `diphthong' in the sense of digraph. Greek diphthonggos `double sound'.

In the English diphthong [ai], the [a] is articulated as an [a], and it glides smoothly into a [i], with the [i] articulated near the position where the vowel [e] is pronounced. The [i] part also has a weaker stress than [a]. In the Japanese [ai], both [a] and [i] have equal stress, are articulated separately with no perceptible glide, and the [i] is articulated exactly at the same position of the monophthong [i]. This is the same for the other 24 possible vowel pairs in Japanese (when not used as long vowels 「長母音」, which are basically just monophthongs with extended vowel lengths), such as:

ああ*／(あい)／あう／あえ／あお
いあ／いい*／いう／いえ／いお
うあ／うい／うう*／うえ／うお
えあ／えい*／えう／ええ*／えお
おあ／おい／おう*／おえ／おお*

(*These are usually just long vowels.)

Hence, the 25 possible vowel pairs are not true diphthongs in any sense of the word. I suppose you could call some of them (the ones that are not long vowels) concatenated vowels 「二連母音」, but this is a mostly pointless distinction as there are for all practical purposes pairs of two monophthongs that happen to occur together.

'Concatenated vowel pairs' are generally recognized to be two separate syllables 「音節」, and an example can be seen here.

英語の母音について種類にわけた表を上に示した。
　日本語の母音が５種類とされているのに対して英語の 母音がいかに多いかが分かる。詳細については後ほど解 説するが、「重母音（２重母音、３重母音）」が「１つ の母音」であるという考え方が重要。それは決して２つ や３つの母音がただ並んでいるというものではない。複 数の母音で１つの「音節」を構成しており、あくまでも それで１拍のリズムを作るというのが重母音の特徴であ る。すなわち日本語の「愛（あい＝２音節）」と英語の「I（私）＝１音節」は同じではない。

Edit: Masaegu, I hope I'm not bothering you, but are there any errors or weird stuff in the translations I attempted in post #50? Thanks in advance, and I'm sorry if it's a bother.