A, called a chen or the great a to distinguish it from the small a, is the thirtieth and last of the consonants and the letter on which the recitation closes.
It is a consonant by position and a vowel carrier in practice. Where a Tibetan syllable begins with a vowel, that vowel is written on a chen, which supplies the shape the vowel sign needs in order to attach to something. With no sign upon it the bare letter is read a, because a Tibetan consonant sign carries an inherent a of its own, so the letter that exists to carry vowels also has one built in.
That makes a chen the letter a learner is shown first when the four vowel signs are introduced, since the signs cannot be demonstrated on their own. It also makes it the base for the transcription of Sanskrit independent vowels, and the base of the syllable that stands for enlightened speech in the three-syllable series.
It is distinct from a chung, the twenty-third letter, in shape, in name and in function, and the two are not interchangeable. Confusing them is the single most common error in reading a stack, because it decides which letter of a syllable is the root.
A chen is not a prefix, a suffix, a superscript or a subscript, and unlike a chung it does no orthographic work at all beyond carrying a vowel. Its whole office is to be a place for a vowel sign to stand, which is why it closes the recitation rather than opening it: it is the letter that exists for the four signs about to be introduced.