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What is a blockchain?

The shared record that everything else runs on.

4 minChecked 4 October 202610 sources

Animation is off, so this lesson is a plain page: one still picture and a few sentences for each step. Turn animation on in the menu to watch it play.

Block 1 sealedFingerprint 9f2c…a41b
Block 1's fingerprint is a made-up example; block 2's is the real SHA-256 of its payment.

Step 1 of 5

Payments are grouped into blocks

A blockchain is a shared record of payments. New payments are grouped into a block, and each finished block gets a fingerprint: a short code calculated from everything inside it, called a hash.

  1. Block 1 sealedFingerprint 9f2c…a41b

    Step 1 of 5

    Payments are grouped into blocks

    A blockchain is a shared record of payments. New payments are grouped into a block, and each finished block gets a fingerprint: a short code calculated from everything inside it, called a hash.

  2. Block 1 sealedFingerprint 9f2c…a41b
    Block 2 stores 9f2c…a41bBlock 1's fingerprint

    Step 2 of 5

    Every block points to the one before

    Each new block stores the fingerprint of the block before it. Follow those links back and you reach the very first block. That is the chain.

  3. Block 1 sealedFingerprint 9f2c…a41b
    Block 3 matchesIt stores 66ed…1719, block 2's fingerprint

    Step 3 of 5

    Change one record and the links break

    Alter an old record and its block gets a different fingerprint. The next block still holds the old one, so the mismatch is plain to see. Try it below.

    Try it

    Block 2's fingerprint
    …
    Block 3 stores
    …

    They no longer match, so block 3's link is broken, and so is every link after it.

    A real SHA-256 fingerprint of the text above, worked out in your browser. Bitcoin fingerprints its blocks with SHA-256, applied twice.

  4. Another computer's copyMatches
    A changed copyDisagrees with everyone else's

    Step 4 of 5

    Thousands of computers keep a copy

    Thousands of computers around the world each keep their own copy and check every new block. A copy that has been changed simply disagrees with everyone else's.

  5. Bitcoin, 2010 and 2013History rewritten after software faults
    Ethereum ClassicAttacked by miners with most of the power

    Step 5 of 5

    What can go wrong

    Hard to change is not impossible. Bitcoin's history was rewritten after software faults in 2010 and 2013, and smaller blockchains such as Ethereum Classic have been attacked by miners who controlled most of the computing power.

Block 1's fingerprint is a made-up example; block 2's is the real SHA-256 of its payment.

Quick check

Did it stick?

2 questions. Get them right to complete the lesson.

Question 1

Question 1 of 2

What does each block store about the block before it?

Question 2

Question 2 of 2

Someone changes a payment in an old block. What happens?

Or skip ahead to lesson 2: Keys and wallets.

Sources

Checked 4 October 2026
  1. Bitcoin developer guide: block chainBitcoin Project
    • A blockchain is a shared record of payments: new payments are grouped into blocks.
    • Alter an old record and its block gets a different fingerprint; the next block still holds the old one, so the link breaks.
    • A copy that has been changed simply disagrees with everyone else's, and the network ignores it.
  2. Bitcoin developer reference: block chainBitcoin Project
    • Bitcoin fingerprints its blocks with SHA-256, applied twice.
    • Each block gets a hash, a short fixed-length code calculated from its contents.
    • Each new block stores the fingerprint of the block before it.
  3. Glossary: cryptographic hash functionNIST Computer Security Resource Center
    • A hash works like a fingerprint: it is computationally infeasible to work back to the input or find two inputs with the same hash.
    • Hard to change is not impossible: rewriting history is computationally infeasible, not impossible.
  4. Bitcoin: A Peer-to-Peer Electronic Cash SystemSatoshi Nakamoto (Satoshi Nakamoto Institute copy)
    • Those computers check every new block and accept it only if its payments are valid.
  5. Ethereum node trackerEtherscan
    • Thousands of computers around the world each keep their own copy (12,105 Ethereum nodes found on 4 October 2026).
  6. Value overflow incidentBitcoin Wiki
    • Bitcoin's history was rewritten after a software fault in 2010.
  7. BIP-50: March 2013 chain fork post-mortemBitcoin Improvement Proposals
    • Bitcoin's history was rewritten after a software fault in 2013.
  8. Attacker stole 807K ETC in Ethereum Classic 51% attackBitquery
    • Ethereum Classic was attacked in 2020 by an attacker who controlled most of its computing power.
  9. 51% attacksMIT Digital Currency Initiative
    • Smaller blockchains have repeatedly been attacked by miners who controlled most of the computing power.
  10. Bitcoin genesis block (block 0)mempool.space
    • Follow those links back and you reach the very first block.

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