What is a Mudra ?

'Mudra' is the unique signature of a Carnatic music composer, which is woven into some or all of their respective compositions especially in the charanam (at the end of the keerthanai).
Not all composers have mudras, and they do not necessarily relate to the composer's name. Mudras are very helpful to find the composer for a particular carnatic krithi.

The mudras for trinity of carnatic music St.Tyagaraja, Muthuswamy Dikshitar and Syama Shastrigal are as follows :

Tyagaraja - Tyagaraja
Syama Shastri - Syama Krishna
Muthuswami Dikshitar - Guruguha

Purandara Dasar leaves a mudra 'purandara vittala' in all his compositions.
Purandara Dasa - Purandara Vitthala

Here are the mudras of some famous composers in carnatic music.

Bhadrachala Ramadas - Ramadasu
Papanasam Sivan - Ramadasan
Gopalakrishna Bharathi - Gopalakrishnan
Harikesanallur Muthiah Bhagavatar - Harikesha
Narayana Teertha - Narayana Teertha
Patnam Subramania Iyer - Venkateshwara
Mysore Vasudevacharya - VasudevA
Mysore V Ramarathnam - rAma
M. D. Ramanathan - Varada dasa
Dr. M. Balamuralikrishna - Hari and Murali
Maharajapuram Santhanam - Maharajan
Koteeswara Iyer - Kavi Kunjara dasa