What Is a Genesis Block?

From Blocks To The First Block

Where does it start?

Position in chain

Always the first block

Parent block

None; it does not point to any earlier block

Main role

Define the initial state and starting rules for the blockchain

Chosen when

At design time, before the blockchain starts running

A paper ledger needs a first numbered page where initial balances are written before any debits or credits are recorded.
A book needs an opening chapter that sets up the world before later chapters can add new events.
A photo album can show changes over time only if there is a clear first picture that captures the starting moment.

The first page

Everything links back

Pro Tip:

The exact contents of the genesis block for a given blockchain are fixed once the chain is launched.
The unique identifier (hash) of that genesis block is fixed and expected by all honest participants.
The fact that the genesis block has no parent block is fixed for that chain.
All honest nodes on that blockchain are expected to start from the same genesis block.

Different blockchains can choose different data to include in their genesis block, such as initial balances or special settings.
The method for assigning initial balances or resources is a design choice and can vary from chain to chain.
The way rules are encoded or referenced in the genesis block is also a design choice and can follow different formats or structures.

Different chains, different starts

Pro Tip:

A blockchain is a shared history made of blocks, and each block is an ordered update to that history.
This history needs a clear starting page, and that starting page is the genesis block.
The genesis block defines the initial state and starting rules for the system.
Every later block links back through previous blocks until the chain reaches the genesis block.
Changing the genesis block would change the entire history that follows from it.
Each blockchain has its own fixed genesis block, chosen when the system is designed.