20. The a is the openest vowel

The a is the openest vowel, an utterance from the expanded throat, stands in no relation of kindred with any of the classes of consonantal sounds, and has no corresponding semivowel. Of the close vowels i and u, on the other hand, i is palatal, and shades through its semivowel y into the palatal and guttural consonant-classes; u is similarly related, through its semivowel v, to the labial class, as involving in its utterance a narrowing and rounding of the lips.

a. The Paninean scheme (commentary to Pāṇini’s grammar i. 1. 9) classes a as guttural, but apparently only to give that series as well as the rest a vowel; no one of thePrātiçākhyas puts a into one class with k etc. All of these authorities concur in calling the i- and u-vowels respectively palatal and labial.