What is a Block?
A collection of Bitcoin transactions that are grouped together and added to the blockchain. A new block is created approximately every 10 minutes. Each block contains a cryptographic reference to the previous block, creating an unbreakable chain of transaction history.
Why It Matters
Blocks are Bitcoin's record-keeping unit. Group transactions together. Link them with cryptography. The result is a tamper-proof ledger.
Try to alter a past transaction and you change that block's cryptographic hash. Which breaks the link to every subsequent block. Which means rewriting Bitcoin's history requires redoing the enormous computational work for every block from the alteration point onward, all while the rest of the network keeps producing new blocks faster than the attacker. Practically impossible. Blocks also limit how many transactions process in a given window. That's why understanding blocks helps you grasp Bitcoin's scalability tradeoffs and why fees rise during congestion.
How It Works
Miners collect pending transactions from the mempool and bundle them into a new block. Each block has a header. The header contains metadata: the previous block's hash (linking the chain), a timestamp, the difficulty target, and a special number called a nonce.
Miners then compete to find a valid nonce that produces a hash meeting the difficulty target. This is where Proof of Work happens. The work is brute-force guessing. Once a miner finds a valid solution, they broadcast the new block. Other nodes verify every transaction inside it and add the block to their copy of the blockchain.
A worked example. Block 840,000, mined on April 19, 2024, is the famous one. It triggered Bitcoin's fourth halving, cutting the block reward from 6.25 BTC to 3.125 BTC. The block contained more than 3,000 transactions and paid 37.626 BTC in fees alone, the highest fee total in Bitcoin history at the time. Any blockchain explorer (mempool.space, blockstream.info) lets you view block 840,000 yourself. You can see every transaction, the timestamp, the miner who solved it, and the cryptographic hash that links it to block 839,999 before it and block 840,001 after.