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The Thampis are the sons of the Maharajahs of Travancore and their morganatic wives belonging to Illathu Nair or other similar Kshatriya castes. They form a part of the Nair caste (even if the mother is a non-Nair, as in the case of the Kunju Thampies) and had no title of succession to the throne.
Since the Maharajahs of South Kerala (Travancore) also belong to the warrior race of the Nairs, they followed the matrilineal system (Marumakkathayam) of inheritance. Hence when a king died his sister's son would become the next Varma (Samantha Kshatriya) ruler and his own son born of his wife, usually of the Illathu Nair caste would be called Thampi with the title "Kannaku Chempakaraman" which was also one of the highest titles of nobility in Travancore.
All of the Maharajahs' female descendants were known by the style of Thankachi added to their name. The sons, as mentioned, were styled Thampis with the title of Kanakku Chempakaraman. Since the Marumakkathayam law of matrilineal inheritance existed in Travancore, the sons of these Thampi gentlemen would, however, not gain any distinguishing title.
Families from where Maharajas got married are known as "Ammaveedus". Arumana Ammaveedu, Vadasseri Ammaveedu, Nagercoil Ammaveedu, Thiruvattar Ammaveedu, etc. are some of the prominent families in Trivandrum from where the Maharajas of Travancore married. The Rajahs of Travancore traditionally took wives from Ammaveedus and such Royal consorts, known as Ammachis would get the additional title of Panapillai Amma. If at all another lady from outside the Ammaveedu's was to be married to the King, she would be adopted to one of the Ammaveedus first and then wed to the King. This was the case in the marriage of Maharajah Swathi Thirunal, Maharajah Ayilyam Thirunal and Maharajah Moolam Thirunal.
As mentioned previously, the title of Thampi was also given to some loyal noble families in Travancore. Thampis had special privileges in Travancore. Besides the Maharajah, they were the only people permitted the use of Palanquins. They also had the right to visit their cousin, their father's heir as per the Marumakkathayam law, without formally previously announcing their visit. The very term Thampi and Thankachi meant, in Tamil language, brother and sister respectively which indicated the position of the Thampi families as relatives of the Royal House of Travancore.
The consort of the ruling prince was known as the Ammachi with the title of Panapillai Amma. To the names of the sons of the Maharajahs was prefixed the title of Sri suffixed with Thampi. The daughters were known as Ponnammas. The other members of the Ammaveedus however were simply known as Thampis and Thankachis or Kochammas.