Pa heads the labial class, the last of the five, made by closing the lips, which is the outermost point of articulation and therefore the last position in an order that runs from the throat forwards. With this class the twenty-five stops are completed.
The Sanskrit phoneticians call the class oshthya, of the lips, and they group the vowels u and long u with it, since those are formed with the lips as well. That cross-reference between the vowel table and the consonant table is one of the clearest signs that the arrangement is a genuine analysis of speech rather than a convenient ordering: the same articulatory categories are applied to both halves of the inventory, and the categories are the point.
The letter is common in the vocabulary, standing in a large number of roots including several that generate substantial families of derived terms, and it appears in a set of very frequent prefixes. It takes the full range of vowel marks and enters conjuncts freely, most often with the semivowel ra and with the sibilants.
Its position at the head of the class is fixed by the same rule as every other class head: voiceless, unaspirated, the plainest treatment of the sound made at that point.