A Bitcoin block may contain thousands of transactions, but its header is only 80 bytes. This tiny piece of data can “lock in” every transaction in the block, thanks to the Merkle tree.

The block header of every $BTC block stores a Merkle root, which corresponds to all the transactions in that block. Currently 82,509.28 (24h +0.89%).

【What Kind of Tree Is It?】

A Merkle tree is a tree-shaped structure that hashes large amounts of data in pairs and merges them layer by layer. At the bottom are the hashes of individual transactions. Each pair of adjacent hashes is concatenated and hashed again to produce the next layer. This continues until only one hash remains, called the Merkle root.

This structure is named after cryptographer Ralph Merkle. It was proposed in the late 1970s, long before blockchain.

【Why changes can’t go unnoticed】

Bitcoin’s block header contains the Merkle root. If any transaction in a block is changed, its hash changes. That change propagates up to the Merkle root, which changes the block hash, making all subsequent blocks no longer match.

So keeping an eye on this one hash in the block header is equivalent to keeping an eye on every transaction in the block.

【How lightweight wallets verify transactions】

To prove that a transaction is in a particular block, you don’t need the entire block—just a few hashes along the path from that transaction to the Merkle root. If the number of transactions doubles, you need only one additional hash. For a block with thousands of transactions, a dozen or so hashes are enough.

This is how Simplified Payment Verification (SPV), described in Section 8 of the Bitcoin white paper, works: a lightweight client stores only block headers, then requests a Merkle path from a node to confirm that a transaction has been included in a block.

【Use in proof of reserves】

Exchanges also use Merkle trees for proof of reserves: the platform puts all users’ balances into a Merkle tree and publishes the root hash. Each user receives their own path and can verify for themselves that their balance has been included, without revealing anyone else’s details.

【Summary】

The idea behind a Merkle tree is simple: use one short root hash to represent a large amount of data. This design is what makes applications like mobile wallets and proof of reserves possible. When I first started out, I thought knowing how to read candlestick charts was enough. Later, I realized that understanding the rules is just the first lesson. Every potential return comes with corresponding risks, so take a closer look at the worst-case scenario.

If you found this useful, save it to look back on later.

Just sharing some notes. Make your own decisions about buying and selling.