Merkle Tree

A Merkle tree is a data structure that summarizes a large set of items in a single hash by repeatedly pairing and hashing: each transaction is hashed, the hashes are paired and hashed together, those results are paired and hashed again, and so on until one value remains — the Merkle root. Blockchains store this root in each block header, committing to every transaction in the block with just 32 bytes.

The structure's power is efficient proof. To show that one specific transaction is included in a block, you don't need the whole block — only the transaction and the handful of sibling hashes along its path to the root, called a Merkle proof. A verifier recomputes the path and checks that it produces the known root; for a block with thousands of transactions, the proof involves only a dozen or so hashes. For example, this is how a lightweight Bitcoin wallet on a phone confirms it was paid: it holds only block headers, asks a full node for the Merkle proof of its transaction, and verifies the math itself — the node cannot fake inclusion because that would require breaking the hash function.

A common misconception is that Merkle trees store or protect the transaction data itself; they only commit to it — the data lives elsewhere, and the tree makes tampering detectable, not impossible to attempt. Variants extend the idea: Ethereum uses Merkle-Patricia tries to commit to its entire account state, and newer designs like Verkle trees aim to shrink proofs further, which matters for making chain verification lighter.