The blockchain is Bitcoin's beating heart: a public ledger that records every transaction ever made, maintained by thousands of independent computers with no single point of control. Understanding it is the key to understanding why Bitcoin works.
The Problem with Ordinary Databases
Banks keep ledgers too. The difference is that their ledger is controlled by them. They decide what's recorded, what's reversed, and who gets access. If a bank makes an error, or a government applies pressure, or a hacker gets in: the ledger can change.
Bitcoin replaces the trusted central database with a distributed ledger: thousands of copies, held by thousands of nodes worldwide, all following the same rules. Changing one copy does nothing: the network ignores it. To rewrite history, you'd need to redo more computational work than the entire rest of the network combined. In practice, this is impossible.
What is a Block?
Transactions don't get added to the Bitcoin ledger one at a time; they get bundled together into blocks. Every ~10 minutes, a new block is produced by a miner. That block contains:
- Several hundred to a few thousand transactions
- A reference (hash) to the previous block
- A timestamp
- The miner's solution to the proof-of-work puzzle
Once a block is added to the chain, the transactions inside it are considered confirmed. Each subsequent block adds another layer of confirmation, making it progressively harder to reverse.
What is a Hash?
A hash is a fingerprint for data. Run any piece of data through a hash function (Bitcoin uses SHA-256) and you get a fixed-length string of characters. The same input always produces the same output. But change even a single character of the input, and the output changes completely and unpredictably.
"Hello" → 185f8db32921bd46d35..."hello" → 2cf24dba5fb0a30e26e8...One letter changed. Completely different output.
This property, tiny change, completely different hash, is what makes the blockchain tamper-evident.
How Blocks Chain Together
Each block contains the hash of the block before it. This is the "chain" in blockchain. If you wanted to change a transaction in an old block, you'd have to:
- Recalculate that block's hash (because the data changed)
- Update the next block, which references the old hash
- Recalculate that block's hash
- And so on, all the way to the current block
- And do all of this faster than the rest of the network is adding new blocks
Given that the network is producing ~144 blocks per day, backed by hundreds of exahashes of computing power, this is computationally infeasible. History is effectively immutable.
Block Height and Confirmations
Block height is simply the number of blocks in the chain since the genesis block (block #0, mined by Satoshi in January 2009). As of early 2026, the chain is over 885,000 blocks tall. Each block represents approximately 10 minutes of the network's history.
Confirmations count how many blocks have been added after the block containing your transaction. One confirmation means your transaction is in the chain. Six confirmations (about an hour) is considered extremely secure for large transactions: an attacker would need to secretly redo six blocks of work to reverse it.
Who Holds the Ledger?
Anyone can run a Bitcoin node. A full node downloads the entire blockchain (currently ~600GB), validates every transaction and block against the rules, and helps propagate new transactions and blocks to the rest of the network. There are thousands of nodes worldwide.
Nodes enforce the rules. Miners produce blocks, but if a miner produces an invalid block, every honest node rejects it. The miner wastes their electricity and gets nothing.
This separation of roles, miners add blocks, nodes enforce rules, is why Bitcoin has no central authority. No single actor controls what is and isn't valid.
Why This Matters for Miners
When you mine Bitcoin, you're not just earning a reward, you're participating in the process that writes the ledger. Each valid block you find adds to the permanent record. Each hash you compute contributes to the security that makes the chain immutable. The more independent miners participating, the more decentralised and attack-resistant the network becomes.