What do you understand by ‘Instrument of Accession’?

Instrument of Accession was a legal document first introduced by the Government of India Act 1935 but executed on 26 October 1947 by Maharaja Hari Singh, ruler of the princely state of Jammu and Kashmir. The instruments of accession executed by the rulers, provided for the accession of states to the Dominion of India(or Pakistan) on three subjects, namely, defence, external affairs and communications.