A record from the sub-registrar of the transactions registered on a property, such as sales and mortgages; often called an EC.
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The papers to ask for, in the order they actually matter.
Ask for it to cover many years, not just the last one. A clean certificate is necessary but does not replace a title check: it cannot show anything that was never registered. In Punjab and Haryana you usually get it from the tehsil, as a non-encumbrance certificate (NEC).