Block Header

Block header is the metadata portion of a block containing fields required to identify, validate, and connect the block to the blockchain.

The exact fields depend on the protocol. A header may contain the hash of the previous block, a timestamp, a transaction-tree root, consensus parameters, difficulty information, or other protocol-specific values.

Linking blocks

A cryptographic reference to the previous block is one of the most important fields. Changing an earlier block changes its identifier and breaks the expected links to subsequent blocks.

In Proof-of-Work networks, header data also participates in the computation used to find a valid block. The header is therefore directly relevant to mining in PoW systems.