Genesis block
The very first block in a blockchain, from which the entire chain begins. Bitcoin's genesis block was created on January 3, 2009.
How it works
The genesis block is the first block of a chain, the only one with no predecessor. Every other block references the one before it by hash, and that chain of references ends here.
It is normally hard-coded into the software rather than mined. A node does not verify it at startup but accepts it as given — there would be nothing to verify against otherwise.
Bitcoin's was created on 3 January 2009, and its data carries a line from a headline in The Times: “Chancellor on brink of second bailout for banks”. That is at once proof the block was not made before that date and a direct statement of why the system appeared.
What is unusual about it
- Its reward cannot be spentThe fifty bitcoins of the first block are not processed through the usual balance mechanism — a quirk of the original code makes them permanently unspendable.
- The address still receives transfersPeople have sent small amounts there for years as a tribute. Those too remain locked.
- The gap before the second blockSix days passed between the first and second block instead of the usual ten minutes. There is still no agreed explanation.
- Every network has oneEthereum, any fork, a private blockchain — every chain begins with a block accepted without verification.
Why it matters
The genesis block defines a network's identity. Two blockchains with identical code but different first blocks are two separate, incompatible networks. This is why a hard fork's new chain keeps the same genesis as the old one: their history matches up to the point of divergence.
One practical consequence follows: trust in a chain starts with trust that your node took the correct first block. Every check after it is meaningless if the starting point was substituted — which is why node software is taken only from an official source and its signature verified.