There are only four varnas - Brahmana, Kshatriya, Vaishya and Shudra. There is no fifth varna. Intermingling of varnas lead to jaatis. Varnasankara refers to intermingling of jaatis, adultery among varnas, prohibited conjugations, negligence of karmas.
Land which lies between the two divine rivers Saraswathi and Drushdvati is created by gods and is called as Brahmavarta. The aachaara that has come from parampara that is practiced in this area by varnas and intermingled jaatis is called sadaachara (conduct of the good). After Brahmavarta, Kurukshetra, Mastya, Paanchaala, Surasenaka are areas of Brahmarshis. Madhyadesha is defined by boundaries - Himvat and Vidhya mountains to the east of Prayaga, West of Vinasana (where the river Saraswathi dissapears) . Aryavart lies in between east and west oceans between these mountains. This region where black antelope (Krishnasaaras) roams naturally is fit for performing yajnas.
Brahmana, Kshatriya, and Vaishya are referred here as dwija varnas. In all varnas, child of a varna with a corresponding akshata-yoni belongs to the same varna as that of parents. Askhata-yoni refers to a woman wedded to a male of same varna as a virgin and bearing children to the same person.
Aryavarta is most conducive for aryas. Those who reside in other areas are mainly mlechhas. A dwija may seek dwelling in Aryavarta. A Shudra, distressed for subsistence, may reside anywhere.
Four varnas, when intermingled, leads to 16 jaatis. Out of these sixteen jaatis, four correspond to four varnas, three jaatis corresponding to dwija varnas. Other three jaatis characterized by offspring generated by a Brahmana with a Kshatriya wife, A Kshatriya with a Vaishya wife, a Vaishya with a Shudra wife are considered as dwija jaatis. Both dwija varnas and dwija jaatis are considered as dwijas. Dwijas have duties of dwija varnas. Other 10 jaatis (viz offspring generated by Brahmana-Vaishya, Brahmana-Shudra, Kshatriya-Shudra, Kshatriya-Brahmana, Vaishya-Kshatriya, Vaishya-Brahmana, Shudra-Vaishya, Shudra-Kshatriya, Shudra-Brahmana, and Shudra-Shudra) have duties assigned to Shudra.
When dwijas do not receive savitri in accordance with rules, they become vratyas. The intermingling of varnas is categorized as anuloma and pratiloma marriages. Anuloma marriage refers to intermingling of men and women in the order of Brahmana, Kshatriya, Vaishya and Shudra. Pratiloma intermingling refers to men and women in the order of Shudra, Vaishya, Kshatriya and Brahmana. Dwijas are a subset of jaatis from anuloma intermingling. Those born out of four varnas - whether speak the language of mlechhas or aryas - are called dasyus. Apasada refers to jaati created due to anuloma intermingling of varnas. Apadvamsaja refers to jaatis created due to pratiloma intermingling of varnas. A bahya is subset of apadvamsaja such as one begotten by a Shudra from a Brahmana woman. Similarly, a bahya begets further bahyas from the females of four varna.