Normal Aperture Back Primary Mid Vowel
Vowels have a wide, firm, and free channel, whereby the breath is modified without friction or sibilation. … in representing vowels the hand suggests a wide and firm channel, by having the accented finger bent and its terminal phalanx brought firmly in contact with the terminal phalanx of the thumb.
Vowel positions are distinguished by always having the voice phalanx of the thumb accented and in contact with the terminal phalanx of the accented finger. This kind of accent is the strongest which can be given a finger, and so always takes precedence. Two modes of accentuation may not co-exist. Back Vowels have the palm in the posterior position. In Primary Vowel positions the accented voice phalanx of the thumb and the terminal phalanx of the accented finger overlap. None of the unaccented fingers are straightened. Mid Vowels have the center finger accented.