Block Header

Definition

A block header is the compact summary of a block that Bitcoin miners hash during proof-of-work calculations. The Bitcoin header is 80 bytes long and includes a version field, the previous block's hash, a Merkle root committing to transactions, a timestamp, an encoded difficulty target, and a nonce. These fields connect a candidate block to a particular history and set of contents. Changing the transaction list changes its commitment and therefore changes the header that miners must hash. A valid proof of work requires the resulting hash to fall at or below the applicable target. Headers make it possible to examine accumulated work without first downloading every transaction, but a header alone does not establish that all transactions obey consensus rules. Full validation requires the relevant block data and the state needed to check its spends.