Block Header

A block header is the compact 80 byte summary at the top of every bitcoin block. It contains six fields: the software version, the hash of the previous block header, the Merkle root committing to all transactions in the block, a timestamp, the encoded difficulty target, and the nonce miners adjust while searching for a valid proof of work.

Why it matters

The header is what actually gets mined. Miners hash the 80 bytes twice with SHA-256, trillions of times per second across the network, hunting for an output below the difficulty target. Because the header includes the previous block's hash, each block is cryptographically welded to its predecessor, and altering any historical transaction would change a Merkle root, break a header, and invalidate every block after it. The chain of headers is therefore the chain of proof of work itself.

Headers also enable light clients. Under simplified payment verification, a wallet can download only the headers, about 4 megabytes per year of chain history, and verify that a transaction is committed in the chain with the most accumulated work, without storing hundreds of gigabytes of full blocks.

How it works

When a candidate block is assembled, the miner computes the Merkle root of its transactions, fills in the other header fields, and iterates the 4 byte nonce. Because the nonce allows only about 4.3 billion values, far too few at modern hashrates, miners also vary the timestamp and the coinbase transaction, which changes the Merkle root and opens a fresh search space.