Semi-vowels and vowel glides. Theoretically, as far as phoneticians are concerned, any segment must be either a vowel or a consonant. If a segment is not a vowel, it is a consonant.
Semi-vowels and vowel glides
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Theoretically, as far as phoneticians are concerned, any segment must be either a vowel or a consonant. If a segment is not a vowel, it is a consonant.
The problematic area is that the initial sound in hot gives little turbulence, depending on how forcefully it is said, and in yet and wet the initial segments are obviously vowels.
To get out of this problem, the usual solution is to say that these segments are neither vowels nor consonants but midway between the two categories.
For this purpose, the terms “semi-vowel” or, less commonly, “semi-consonant”, are often used.
Languages also frequently make use of a distinction between vowels where the quality remains constant throughout the articulation and those where there is an audible change of quality. The former are known as pure or monophthongvowels and the latter, vowel glides.
If a single movement of the tongue is involved, the glides are called diphthongs.
Diphthongal glides in English can be heard in such words as way , tide , how , toy , and toe .
A double movement produces triphthongs.
A triphthong is “a glide from one vowel to another and then to a third, all produced rapidly and without interruption” (Roach, 2000: 24).
They are really diphthongs followed by the schwa [ə], found in English words like wire  and tower .
The vowels of English (RP)
There are certain differences between the qualities of vowels in RP and GA.
Notably, the central vowels  are r-colored or rhotic for GA, transcribed as  and , respectively. It means that they involve curling the tip of the tongue up in a gesture of retroflection and the phenomenon is known as r-coloring or rhoticity.
Another major difference is that in GA  is used where it is  in RP and  replaces  in RP.
Other minor differences exist but they do not lead to noticeable effects to the ordinary perception.
The description of these vowels needs to fulfill four basic requirements:
the height of tongue raising (high, mid, low);
the position of the highest part of the tongue (front, central, back);
the length or tenseness of the vowel (tense vs. lax or long vs. short), and
lip-rounding (rounded vs. unrounded).
Now we can describe the English vowels in this way: