block header

A block header is the small summary section at the top of every block — the cover sheet that describes the block without listing all its transactions. While a block's body may hold thousands of transactions and be quite large, its header is tiny and fixed in size, holding just a handful of essential fields. This compactness is deliberate: it lets the network identify, verify, and link blocks quickly without dragging around the full contents each time.

The header packs in the few facts that make the chain hang together. It contains the hash of the previous block's header (the link backward that forms the chain), a single fingerprint summarizing all the transactions inside (the Merkle root, like one number that vouches for the entire receipt book), a timestamp, and the rule-specific values the network uses to accept the block — for example, on a proof-of-work chain, a difficulty target and a nonce. Hashing just this small header, rather than the whole block, is enough to commit to everything.

Headers matter because they let lightweight devices trust the chain without storing all of it. A phone wallet can download just the chain of headers — a fraction of the full data — and still confirm that a given transaction was included, by checking it against the Merkle root in the relevant header. The header is the handle by which the entire heavy block can be safely grasped, referenced, and proven.