Semi-Vowels
In English, a short version of [i], spelt y, occurs in yet and a short version of [u], spelt w, occurs in west. If you say yet giving the y the length of an ordinary vowel, you’ll notice that it is in fact the same as [i] in quality (high, front, unrounded). Similarly with the w of west (high, back, rounded.)
These truncated high vowels are, for obvious reasons, known as semi-vowels. They always occur at the beginnings or ends of syllables, just as consonants do, never as the centre of a syllable: thus met, pet, set (consonants) and yet (semi-vowel). In short, semi-vowels are sounds which are articulated like vowels but positioned in words like consonants. (Indeed the term semi-consonant is occasionally applied to them.)
The symbol [w] has been adopted unchanged by the IPA to represent the high back semi-vowel of west ([wEst]). However, the [y] symbol isn’t available for yet, as it’s already in use for Cardinal 9 (the vowel in rue and über). Instead [j] is used: so yet is transcribed [jEt]. Like [y], [j] in the IPA has the same value as in German spelling (cf. Jahr, etc.). But it’s not called “jay”, as this would be too suggestive of the sound it has in English. Instead you should read it as “yod”. [w] on the other hand is still read“double-you”.
A point to note about [w] relates to words like when, which, whether. In some accents (notably American, Scots, and conservative RP), [w] in such cases becomes voiceless: the wh in the spelling is an attempt to represent this, in the absence of any distinctive letter. A special symbol has been devised for the IPA, however — an inverted w: [˜]. So for some English speakers there is a difference between [wain] (wine) and [˜ain] (whine), or [weilz] (Wales) and [˜eilz] (whales).
French specialists should know that as well as [j] and [w], French (unlike English, German or Spanish) has a third semi-vowel — a shortened version of [y]. The phonetic symbol is [] (“turned h”). This is the sound that is usually represented in ordinary French spelling by u when followed by another vowel, e.g. in puis or nuage. It should be distinguished from [w], which corresponds to ou in the spelling. So there’s a difference between Louis [lwi] and lui [li], and between “bury oneself
— s’enfouir, with [w]: [sfwir] — and “run away” — s’enfuir, with []: [sfi]. The best way to practise a word like puis is to start by pronouncing it with two full vowels [py] + [i] and gradually shorten the [y] so that you end up with a word of one syllable, containing a semi-vowel and just one vowel: [pi]. But make sure you’re pronouncing the [y] as [y] and not as [u]: otherwise you’ll be saying [pwi]!
Notice also that [j] can occur at the end of words in French: travail [travaj], pareil [parj], grenouille [grnuj]. [aj, j, uj] aren’t diphthongs: in [aj] for example there is a rapid transition from low [a] to high [j], with minimal time spent on the intervening stages. This is quite unlike the much more “drawn out” diphthong of English high, with its gradual transition. (The case of haï — mentioned in Exercise 42c — is different again: this word has two syllables each consisting of a full-length vowel [a +i], and the same amount of time is spent on each.)