WHY: SPV allows a user to verify that a transaction is in the Bitcoin blockchain, without running a full node. A lightweight SPV client only needs the block headers of the longest chain instead of full blocks.
To verify that a transaction is in a block, a SPV client requests proof in the form of a Merkle branch. “Linking the transaction to a place in the active chain demonstrates that a network node has accepted it, and blocks added after it further establish confirmation.” (R1)
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