Genesis block

The genesis block is the block at height (index) zero and the single exception to the rule that chains a blockchain together. Every other block includes the hash of the one before it, but the genesis sets that field to zeros because no predecessor exists. Unlike the rest, it is not produced by the normal mining or network validation process: it is hardcoded into the node software, in the source code or a configuration file. Every node that boots the network starts from exactly the same block zero, and that defines which chain is the chain and tells it apart from any other.

Its legitimacy comes not from a prior consensus but from every participant running software that contains the same genesis. It usually holds the network's starting state: in account-and-balance models, the initial token allocations are written into it; in UTXO chains it can include an initial coinbase transaction. In Ethereum, block zero is parameterized through the genesis.json file, with fields such as alloc for initial balances, difficulty, gasLimit and timestamp. From block one onward, blocks are produced under the normal consensus rules (proof of work, proof of stake) and each one points by hash to the previous one all the way back to the genesis.

The genesis is the root of trust and the anchor point of the entire chain, and it defines the network's identity. It is immutable and permanent, never reorganized or replaced. Bitcoin's block zero was mined on 3 January 2009, and its 50 BTC coinbase output is permanently unspendable; it embeds the headline The Times 03/Jan/2009 Chancellor on brink of second bailout for banks, proof that it was not created before that date. Ethereum's block zero, from the Frontier network (30 July 2015), remains in force despite multiple hard forks and the 2022 transition to proof of stake. Anyone launching a new network defines their own genesis, and that is the technical act that founds it.