Na is the twelfth of the thirty consonants and the nasal that closes the dental line. It writes a dental nasal, made with the tip of the tongue at the teeth, and as an initial it takes low tone in modern Lhasa speech, in company with the other nasals of the recitation.
It is one of the ten suffixes and closes an enormous number of Tibetan syllables, which makes it one of the letters an eye meets most often. It is not among the five prefixes, so its orthographic work is done entirely at the end of a syllable.
That work is not merely to sit there. In the modern language a final na alters the vowel in front of it, so that the written vowel and the heard vowel differ in a way that depends on the suffix rather than on the vowel sign. A reader therefore cannot take the vowel from the sign alone and add the consonant afterwards. The syllable has to be taken whole.
Na is also, with ra, la, sa and the small a, one of the suffixes that carry grammar rather than lexicon. A great many verb endings and particles of written Tibetan end in it or consist of it, so a reader who has learnt the letter as a sound will meet it far more often as a piece of sentence structure.
With na the third of the four regular lines closes. After it the labials run the same pattern through for the fourth and last time, and the recitation then moves into the part of the alphabet that Tibetan built for itself rather than inherited.