The brahmans, ksatriyas, and vaisyas are dvija, twice-born, because they have a second, spiritual birth when they are invested with the sacred thread. In terms of caste, jati is the social stratum in which one is born. One is fixed in a jati by birth and there are sets of rules governing acceptable occupations, foods, marriage, and association with other jatis. Though there are only four varnas, there are thousands of jatis. The relationship between the varnas and jatis is complex.
Some scholars believe the varnas evolved into the jatis.
However, there is controversy over the varnas, whether they were marriage-restrictive and whether a considerable proportion of the population lived completely outside the varna structure. But many orthodox Hindus believe the original varnas are still in place and serve to express divine justice through the transmigration of souls. The varna-jati one is born into reflects the reward or punishment of each soul for its previous life. All the jatis of the brahmans link into the all-India brahmanavarna, which is the classical varna. The actual rank of a jati is not so much due to social and occupational practices of the members but is more because of the purity of the souls of the members resulting from their previous lives.