Block header

The block header is a metadata structure which is used to link blocks in the blockchain.

Block header has a size of 80 bytes and consists of 6 fields:

Field	Type	Size	Description	Updated when
nVersion	unsigned int	4	Block header version	You upgrade the software and it specifies a new version
hashPrevBlock	unsigned char[]	32	Previous block hash, used to link block headers into list	New block is accepted
hashMerkleRoot	unsigned char[]	32	Merkle tree hash, used to link block header and block contents	Transactions pool is updated
nTime	unsigned int	4	Block header unix timestamp	Every few seconds for proof-of-work, every successful attempt for proof-of-stake
nBits	unsigned int	4	Compact representation of claimed proof difficulty	The difficulty is adjusted
nNonce	unsigned int	4	Attempts counter for proof-of-work	New proof-of-work hash tried, or never with proof-of-stake

Each block header is required to satisfy the claimed proof.