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Blocks and the chain

9 min read

A block is a batch of transactions with a header that points at the block before it. That pointer is what makes history expensive to rewrite.

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Answer every question correctly (100%) to complete this lesson.

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A block has two parts: an 80-byte header and a list of transactions. The header contains the version, the hash of the previous block header, a Merkle root summarising every transaction in the block, a timestamp, the difficulty target and a nonce.

The Merkle root means you cannot alter a single transaction without changing the root, and you cannot change the root without changing the header hash. Because each header commits to the previous header hash, altering any historical block invalidates every block after it.

Rewriting history therefore means redoing all the proof of work from that point forward, faster than the rest of the network is extending the honest chain. That is the economic barrier, and it grows with each confirmation.

Explain more simply

Transactions are collected into batches called blocks, roughly every ten minutes.

Each block carries a fingerprint of the previous block. Change an old block and every later fingerprint breaks, which everyone would notice immediately.

Real-world analogy

A chain of wax seals: each seal is pressed over the previous one. Breaking an old seal means re-pressing every seal after it, while everyone watches.

Key facts

  • A block header is 80 bytes.
  • The consensus limit is 4,000,000 weight units per block.
  • Confirmations are just the number of blocks built on top of the one containing your transaction.

Common misconception

“Data in a block is encrypted.”

Blocks are public and unencrypted. Bitcoin uses hashing and digital signatures, which prove integrity and authorisation, not secrecy.[1]

Go deeper

Block weight, not raw size, is the consensus limit: a block may not exceed 4,000,000 weight units, where witness data counts a quarter as much as non-witness data (BIP-141).

Nodes also enforce timestamp rules, a coinbase maturity of 100 blocks, and a limit on signature operations. 'Longest chain' in practice means the chain with the most cumulative work, not the most blocks.

Quick check

Answer every question correctly (100%) to complete this lesson.

  1. 1.What does a block header commit to?

    What does a block header commit to?

  2. 2.Why do more confirmations mean more security?

    Why do more confirmations mean more security?

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Extra exam questions

Every question here counts towards your accuracy, XP and rank. No guessing: every answer is explained.

16 questions

Quick check

Answer every question correctly (100%) to complete this lesson.

  1. 1.What is a Bitcoin block?

    What is a Bitcoin block?

  2. 2.How are blocks linked to form a chain?

    How are blocks linked to form a chain?

  3. 3.How large is a Bitcoin block header?

    How large is a Bitcoin block header?

  4. 4.What is the approximate target time between blocks?

    What is the approximate target time between blocks?

  5. 5.What structure efficiently summarizes all transactions in a block?

    What structure efficiently summarizes all transactions in a block?

  6. 6.Where is the Merkle root stored?

    Where is the Merkle root stored?

  7. 7.What happens if someone tries to alter a transaction in an old block?

    What happens if someone tries to alter a transaction in an old block?

  8. 8.What is the first transaction in every block called?

    What is the first transaction in every block called?

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Sources