In Hinduism there exists four castes arranged in a hierarchy. Anyone who does not belong to one of these castes is an outcast. The religious word for caste is 'Varna'. Each Varna has certain duties and rights. Each Varna members have to work in certain occupation which only that Varna members are allowed. Each Varna has certain type of diet. The highest Varna is of the Brahman. Members of this class are priests and the educated people of the society. The Varna after them in hierarchy is Kshatria. The members of this class are the rulers and aristocrats of the society. After them are the Vaisia. Members of this class are the landlords and businessmen of the society. After them in hierarchy are the Sudra. Members of this class are the peasants and working class of the society who work in non-polluting jobs. The caste hierarchy ends here. Below these castes are the outcasts who are untouchable to the four castes. These untouchables worked in degrading jobs like cleaning, sewage etc.
The first three castes had social and economical rights which the Sudra and the untouchables did not have. The first three castes are also seen as 'twice born'. The intention in these two births is to the natural birth and to the ceremonial entrance to the society at a much later age.
Each Varna and also the untouchables are divided into many communities. These communities are called Jat or Jati (The caste is also used instead of Jat). For example the Brahmans have Jats called Gaur, Kokanastha, Sarasvat, Iyer and others. The outcasts have Jats like Mahar, Dhed, Mala, Madiga and others. The Sudra is the largest Varna and it has the largest number of communities. Each Jat is limited to professions worthy of their Varna. Each Jat is limited to the Varna diet. Each Jat members are allowed to marry only with their Jat members. People are born into their Jat and it cannot be changed.
This is the how the caste system is supposed to be in its religious form. But in reality it is much more complicated and different from its religious form.
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