A seal applied to a document or object in response to or to authenticate another seal; a counter-signature in seal form.
From counter- + seal (from Old French seel, from Latin sigillum). A governmental and legal term for the practice of multiple sealing authorities.
Medieval and early modern documents often required counterseal—a lord would seal a document, but it needed counterseal from a higher authority to be fully official. This created redundancy and prevented forgery, a principle still used in authentication today.
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