Three Nasals Where Indo-Aryan Has Two

Tamil holds an alveolar nasal apart from the dental and the retroflex nasals that the inherited Brahmi order already supplied, so the language needs three signs where Indo-Aryan needs two, and the third was added at the end of the order. The distinction is real in ordinary vocabulary and the three sounds separate words, which is why a script serving Tamil could not simply reuse an existing letter and let context decide.

The letter has a restricted distribution. It does not begin words in the way the dental nasal does, and it is most often met inside and at the end of them, and in a cluster with the additional alveolar r that Tamil-Brahmi also supplied. That combination is one of the characteristic sequences of the language and it involves two letters that exist nowhere in the northern alphabets, so a word containing it could not be written in Brahmi as received from the north at all.

Its position after the retroflex lateral that closes the inherited consonants marks it as an addition, in the standard way. Where a letter sits in an Indic alphabet is often a fact about its history rather than about its sound, and the tail of the order is where a script records what it had to go and get.