Block Header

Overview

The block header allows nodes to identify and validate key properties of a block efficiently. Bitcoin’s header is 80 bytes and contains the version, previous block hash, Merkle root, timestamp, encoded target, and nonce. Its hash serves as the block identifier and proof-of-work value.

Other blockchains use different header structures. An Ethereum execution payload header commits to transactions, receipts, state, gas use, fees, and other fields, while the beacon block includes consensus-layer data.

Headers are useful for lightweight verification because a client can download a chain of headers without downloading every transaction. A valid header chain alone, however, cannot prove that all transaction and state-transition rules were followed. Full validation requires the block body and relevant state data.

The header links each block to its parent and commits to the block’s contents through root hashes. If a committed transaction changes, the corresponding root and block hash also change.

Headers support efficient chain verification. Lightweight clients may follow a sequence of valid headers before downloading relevant transactions or blocks, while full nodes use them as part of complete validation. In Bitcoin, changing any transaction changes the Merkle root and therefore the block header hash.

A header should not be treated as proof that every transaction was independently checked by the application reading it. The application still needs to validate the relevant inclusion proof, chain rules, or trusted node response. Header timestamps may also be approximate rather than exact wall-clock records. Developers should consult the target protocol’s specification instead of assuming that all chains use Bitcoin-style header fields.