The sobriquet (the Land of the Seven Sisters), had been originally coined to coincide with the inauguration of the new states in January, 1972, by Jyoti Prasad Saikia, a civil servant from Assam in the course of a radio talk show. He later compiled a book on the interdependence and commonness of the Seven Sister States, and named it the Land of Seven Sisters. It has been primarily because of this publication that the nickname has caught on.
While factually, one can say that this theory holds true, the reason supporting this is the similarity between the states, in terms of geographical features, and interdependence between them. All the seven states are isolated from India and the only way to reach there is via Siliguri Corridor (also called Chicken’s Neck) in Assam. Tripura is like an enclave surrounded by Bangladesh and depends on Assam for transportation. All of the rivers that flood the plains in Assam originate in Arunachal Pradesh and Nagaland. Mizoram and Manipur are connected to the rest of India through Barak Valley in Assam. And it is due to this interdependence, they were given the sobriquet.
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