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Simple Payment Verification is a system outlined in the original Bitcoin Whitepaper that enables light clients (wallets running on low-end systems) to verify that a transaction has been included in Bitcoin and therefore a payment has been made. When Satoshi designed Bitcoin, he used a data structure called Merkle tree to store the transactions in each block. Someone that only knows the merkle root/top hash can verify if a transaction is part of the tree, that is, if it’s been included into a Bitcoin block.