In this lesson you will learn to:
- Describe what a block contains.
- Explain how blocks are linked by hashes.
- Explain why tampering with old blocks is detectable.
A block is a page in the ledger
Transactions are not added to a blockchain one at a time. They are grouped into blocks, like lines on a page of an account book. Each block has two parts:
- a body: the list of transactions, and
- a header: a small summary that includes the hash of the previous block, a fingerprint of all the transactions in this block, a timestamp and fields used by the consensus method.
The chain: each block points back
Because every header contains the hash of the block before it, the blocks form a chain stretching back to the very first one, often called the genesis block. Bitcoin's genesis block was created on 3 January 2009.
Now recall the avalanche effect from lesson 2.1. Suppose someone changes a single transaction in an old block. That block's fingerprint changes, so its header hash changes, so the next block's "previous hash" no longer matches — and neither does any block after it. The tampering is instantly visible to anyone who checks.
Why not just recompute everything?
A determined attacker could try to recompute the altered block and every block after it. On a public blockchain, that would mean redoing the consensus work for all those blocks faster than the rest of the network keeps adding new ones (lesson 3.3). The deeper a block is buried, the more impractical this becomes. This is why people say blockchain data is immutable — more precisely, it becomes harder and harder to change over time.
The idea of linking records with hashes to make them tamper-evident was described by Stuart Haber and W. Scott Stornetta in 1991, years before blockchains, for time-stamping digital documents. The Bitcoin whitepaper cites their work.
Summarising many transactions: Merkle trees
A block may contain thousands of transactions. Rather than hashing them in one long list, they are paired and hashed, then the results are paired and hashed again, and so on, until a single hash remains: the Merkle root, named after Ralph Merkle. Only this root goes into the header.
The clever part is that anyone can prove one transaction is in a block by showing just a handful of hashes along its branch, without downloading every transaction. Lightweight software on phones relies on this.
How often are blocks added?
Each blockchain sets its own rhythm. Bitcoin aims for roughly one block every ten minutes and adjusts difficulty every 2,016 blocks to keep that pace. Ethereum produces a block slot every twelve seconds. Faster is not automatically better: shorter intervals make it harder for a worldwide network to stay in step.
Key takeaways
- A block bundles transactions with a header that summarises them.
- Each header contains the hash of the previous block, forming a chain.
- Changing an old block breaks every link after it, so tampering is visible.
- Merkle trees let a single hash represent and prove thousands of transactions.
Check yourself
What links one block to the previous one?
The header of each block contains the hash of the previous block.
What is a Merkle root?
The single hash at the top of a tree of hashes that summarises all the transactions in a block.
Why do older blocks become harder to change?
Changing one would require redoing the consensus work for it and every later block, faster than the network adds new ones.
Sources
This lesson is for general education about how the technology works. It is not a recommendation to acquire, sell or hold any crypto-asset and is not financial advice.