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Illam // is a Malayalee lineage system used for classification and identification of castes in Kerala, South India. There are mainly two castes which used Illam as their house. They are Thiyya  and Namboothiris. The House of Namboothiries (the Brahmins of Kerala) are also called the Brahmaalayam or Mana. The house is built according to the canons of Vaasthusaasthram, meaning architecture in Sanskrit language.
In Thiyya system there are eight illam from where they are believed to originate. The history book "Castes and Tribes of Southern India" by Edgar Thurston (1855-1935) describes the eight illams of Thiyya, to which Theeyyars belong:
- Vilakkan Kudi
Ezhavas have no such illam. Marriage between individuals of same illams is prohibited and it is still strictly followed in North Malabar.
In Namboothiri system, each Namboothiri male (or unmarried female) is identified by his/her respective paternal family name. A married female adopts her husband's family name. Each family is affiliated to a Gothra and Pravara. The Gothra name demonstrates the family's traditional style of knowledge acquisition and expertise in ancient theories. Based on the fact that cross-breeding of excellent but different species yields better quality, marriage from a family belonging to the same Gothra was and is still banned for Namboothiris. Each Gothra has several sub-classes known as Pravara. If by mistake, a boy marries a girl of the same Gothra, he is not allowed to have sex with her. He has to treat her like he treats his mother.
Common gothra (and their pravara in brackets) among Namboothiris are:
- Bharadwaajam (Amgirasam, Bhaarhaspathyam, Bharadwaajam)
- Kousikam (Viswaamithram, Akhamarshanam, Kousikam)
- Vaatsam (Bhaargavam, Chyavanam, Aapthavaanam, Ourvam, Jaamadagnyam)
- Koundinyam (Vaasishtam, Maithraavarunam, Koundinyam)
- Kaasyapam (Kaasyapam, Aavatsaaram, Naidruvam)
- Vaasishtam (Vaasishtam, Indrapradamam, Aabharaswath)
- Jaamadagnyam (Bhaargavam, Chyavanam, Aapthavaanam, Ourvam, Jaamadagnyam)
- Viswaamithram (Viswaamithram, Devaraatham, Oudalam)
- Gouthamam (Amgirasam, Aayasyam, Gouthamam)
- Athri (Aathreyam, Archanaanasam, Syavaaswam)
Eight more Gothras also exist among Namboothiris, as branches of the Gothras listed above. They are: Kutsam, Mudgalam, Aamgirasam, Gaargyam, Naidruvam, Saandilyam, Dhaananjayam, and Samkhyaayanam.