Gotras and pravaras

Brahmins classify themselves on the basis of their patrilineal descent from a notable ancestor. These ancestors are either ancient Indian sages or kshatriyas (warriors) who chose to become Brahmins. The ten major gotras that trace descent from sages are: Kanva, Jamadagni, Bharadvâja, koundinya, Gautama, Srivatsasa, Vashista, Atryasa, Kashyapa, Agastya gotra. Two gotras that trace descent from kshatriyas are Mitra and Vishvamitra gotra.
In general, gotra denotes any person who traces descent in an unbroken male line from a common male ancestor. Pāṇini defines gotra for grammatical purposes as 'apatyam pautraprabhrti gotram' (IV. 1. 162), which means: "the word gotra denotes the progeny (of a sage) beginning with the son's son". When a person says, "I am Kashypasa-gotra", he means that he traces his descent from the ancient sage Kashyapa by unbroken male descent. This enumeration of eight primary gotras seems to have been known to Pāṇini. These gotras are not directly connected to Prajapathy or latter brama. The offspring (apatya) of these Eight are gotras and others than these are called 'gotrâvayava'.
The gotras are arranged in groups, e. g. there are according to the Âsvalâyana-srautasûtra four subdivisions of the Vashista gana, viz. Upamanyu, Parāshara, Kundina and Vashista (other than the first three). Each of these four again has numerous sub-sections, each being called gotra. So the arrangement is first into ganas, then into pakshas, then into individual gotras. The first has survived in the Bhrigu and Āngirasa gana. According to Baudh., the principal eight gotras were divided into pakshas. The pravara of Upamanyu is Vashista, Bharadvasu, Indrapramada; the pravara of the Parâshara gotra is Vashista, Shâktya, Pârâsharya; the pravara of the Kundina gotra is Vashista, Maitrâvaruna, Kaundinya and the pravara of Vashistas other than these three is simply Vashista. It is therefore that some define pravara as the group of sages that distinguishes the founder (lit. the starter) of one gotra from another.
There are two kinds of pravaras, 1) sishya-prasishya-rishi-parampara, and 2) putrparampara. Gotrapravaras can be ekarsheya, dwarsheya, triarsheya, pancharsheya, saptarsheya, and up to 19 rishis. Kashyapasa gotra has at least two distinct pravaras in Andhra Pradesh: one with three sages (triarsheya pravara) and the other with seven sages (saptarsheya pravara). This pravara may be either sishya-prasishya-rishi-parampara or putraparampara. Similarly, Srivatsasa gotra has five sages or is called Pancharsheya and are the descendants of Jamadagni. When it is sishya-prasishya-rishi-parampara marriage is not acceptable if half or more than half of the rishis are same in both bride and bridegroom gotras. If it is putraparampara, marriage is totally unacceptable even if one rishi matches.