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1.                 From English-Hindi FreeDict Dictionary ver. 1.6:
                   
   
   
                   
                     
   keystone /kˈiːstəʊn/ <N>
   1. मुख्य~शिला
         "The big white stone in the centre was the keystone of the monument."
   2. प्रधान~सिद्धान्त
         "Doing things honestly has been a keystone of his life."