What Is A Block

A block is a batch of Bitcoin transactions grouped together and permanently added to the blockchain. A new block is found roughly every ten minutes. Each one contains several specific components.

The transaction list. The main content of any block — typically several thousand transactions that have been waiting in the mempool (the queue of unconfirmed transactions). The miner who found the block selects which transactions to include, generally prioritising those with the highest fees.

The coinbase transaction. The very first transaction in every block is special — it creates new Bitcoin out of nothing and sends it to the miner who found the block. This is the block reward — currently 3.125 Bitcoin per block after the 2024 halving. It’s the only mechanism by which new Bitcoin ever enters existence.

The block header. A compact summary of the block containing: the hash of the previous block (the chain link), a timestamp, a difficulty target, and a nonce — a number miners adjust repeatedly while searching for a valid hash.

The Merkle root. A single hash that summarises all the transactions in the block. Any change to any transaction changes the Merkle root, which changes the block header, which breaks the chain. It’s an efficient way to verify that nothing in the block has been altered.

The genesis block — Bitcoin’s first block, mined by Satoshi on January 3, 2009 — embedded a message in its coinbase transaction: “Chancellor on brink of second bailout for banks.” That message is still there. Every copy of the blockchain in the world contains it.

Every ten minutes, a new block. Every block, a permanent page in a book nobody can edit.

Tomorrow: what is a chain — why blocks link and why that makes everything tamper-proof.

— The Daily Bit