What is the Tamil letter missing in thirukural?

There isn't a single Tamil letter that's "missing" from Thirukkural. The Thirukkural is written using the Tamil alphabet, which includes all the necessary letters to write the entire text.

It's possible you might be thinking about a different aspect of Thirukkural or Tamil writing, like:

* Specific sounds: While Thirukkural uses all the letters of the Tamil alphabet, there might be sounds that are rarely used in the text.

* Letter combinations: Thirukkural might not utilize every possible combination of letters within the Tamil alphabet.

* Historical changes: The Tamil language has evolved over time, and some letters might have been used differently in the past compared to today.

If you can provide more context about your question, I can give you a more specific and accurate answer!