Block Header

A block header is a compact, fixed-size data structure sitting at the top of a block that lets nodes and lightweight clients verify a blockchain's integrity without downloading or replaying every transaction it contains. On Bitcoin, the header is exactly 80 bytes, which is why block explorers and simplified payment verification (SPV) wallets can sync an entire chain history in megabytes rather than hundreds of gigabytes.

Bitcoin's header packs six fields: a version number signaling which consensus rules apply, the hash of the previous block's header (the pointer that literally chains blocks together), the Merkle root summarizing every transaction in the block, a timestamp, the compact difficulty target, and the nonce miners cycle through while searching for a valid proof of work solution. Because the previous-block-hash field ties each header to its parent, altering any historical transaction changes that block's hash and breaks every header after it, forcing an attacker to redo the work for the whole remaining chain.

Account-based networks like Ethereum extend the concept further: their headers also commit to a state root, receipts root, and gas usage figures, since validators must agree not just on which transactions occurred but on the resulting account balances and contract storage.

Because the header alone is enough to confirm a block's proof of work and its transactions' authenticity, it underpins:

• Light and mobile wallets that trust the longest valid header chain
• Merkle proofs, which let a single transaction be verified against the header's Merkle root without the full block
• Cross-chain bridges and oracles that check headers to confirm events happened on another chain