genesis block

The genesis block is the very first block of a blockchain — block number zero, the root from which the entire chain grows. Every other block points back to the one before it, but the genesis block has nothing before it, so it is the one block that cannot reference a predecessor. It is the anchor: the fixed starting point that the whole tower of later blocks ultimately rests on.

Because there is no earlier block to derive it from, the genesis block is usually hard-coded directly into the software that runs the network. Everyone who joins agrees on the exact same genesis block down to the last byte; if two people disagreed about block zero, they would simply be on two different blockchains. In this sense the genesis block also defines the network's identity — it is the shared origin that every honest participant accepts by default.

Genesis blocks are often given small commemorative touches by their creators, since they are written by hand rather than mined in the ordinary way. Bitcoin's genesis block famously embeds a newspaper headline from January 2009 in its data, both as a timestamp proving it could not have been made earlier and as a quiet statement of purpose. Symbolic or plain, the genesis block's real job is the same everywhere: to be the undisputed first link that gives every later block something solid to chain onto.