Many of the parameters for consonants are also relevant for vowels (e.g., state of the glottis, nasality). Even the basic constriction parameters can be relevant:

Since vowels are partly defined by the highest point reached by the tongue body, it makes sense to think of the tongue body as the active articulator. For high vowels, the tongue body is narrowing the vocal tract, so it makes sense to ask what passive articulator the narrowing is closest to and what the degree of constriction is.

Elliptical vowel space

For high front vowels, the tongue body approaches the hard palate. For high back vowels, the tongue body approaches the soft palate. The vocal tract is narrowed, but not so much as to cause the airflow to become turbulent -- so the degree of constriction is approximant.

If we wanted to use the consonant parameters for [i] and [u], we would have to say:

But we already have a dorso-palatal approximant -- [j]. We already have a dorso-velar approximant as one half of [w] (it also occurs alone in many languages and is represented by [turned-m-with-tail]).

In fact, the tongue body positions can be the same for [i] and [j]. The only difference is in whether the sound is acting like a vowel (standing in the centre of a syllable) or like a consonant (standing near the edges of a syllable). Glides are vowels masquerading as consonants.

A [j] isn't necessarily exactly like [i] -- English only seems to care that the tongue position is somewhere in the high or upper mid region:

For any dorsal approximant acting as a consonant, we could be very precise about the position of the tongue body by using the appropriate vowel symbol -- we could emphasize the fact that it's acting like a consonant by putting a semicircle (the diacritic meaning "non-syllabic") under the vowel. In a broader transcription, we usually want to ignore details like these and simply use [j] or [w] as a cover symbol.

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