Nna is the retroflex nasal and closes the third class. In classical Sanskrit most of its occurrences are not lexical at all: they are produced by a rule that converts the dental nasal into this one when certain retroflex or r-like sounds have occurred earlier in the same word, sometimes at a distance of several syllables.
A reader who knows the rule can very often predict the letter, and a scribe who does not know it writes the wrong nasal. That is a different kind of literacy from recognising shapes, and it is one reason the study of the script in East Asia was inseparable from the study of the grammar standing behind it. A manual of the letters that did not also teach the rules would produce copyists who could draw the syllabary and not write the language.
The letter closes its class as the other four classes are closed, by the nasal made at the point of articulation the class is named for. Taken together the five nasals are among the most durable results of the Indian analysis of speech, and they are the part of the table that most clearly cannot be reproduced by any Mediterranean alphabet.
On the page it enters conjuncts freely with the stops of its own class, and it takes the vowel marks and the inherent short a like any other consonant sign.