TERM_DEF // BITCOIN_BASICS / BLOCKCHAIN
BLOCKCHAIN
Blockchain. An append-only chain of blocks where each block commits to the previous one via its hash, making history tamper-evident.
This page sits in the Bitcoin Basics section — The vocabulary you meet on day one — what Bitcoin is and the pieces that make it move. Read on for what it is, why it exists, how it works under the hood, and what to watch out for.
This page sits in the Bitcoin Basics section — The vocabulary you meet on day one — what Bitcoin is and the pieces that make it move. Read on for what it is, why it exists, how it works under the hood, and what to watch out for.
WHAT_BLOCKCHAIN_IS
Blockchain — at a glance
BITCOIN BASICS
The blockchain is Bitcoin's append-only public ledger: an ordered sequence of blocks, each one cryptographically pinned to the block before it via a hash pointer. Roughly every ten minutes a new block of transactions is added; the chain only grows in one direction, and changing anything in any past block invalidates every block built on top of it. That is the whole trick that lets thousands of mutually-distrusting computers worldwide agree on a single history of who paid whom — with no bank, no government, and no admin.
Why it exists
DESIGN
Digital cash had a single open problem for thirty years: the double-spend problem. A purely digital coin is just a number, so what stops someone from sending the same coin to two people at once? Every prior attempt answered "a trusted server keeps a ledger" — and then the server became the single point of failure (Mt. Gox, every payment processor, every bank). Bitcoin's blockchain answers the question without a server: the ledger is replicated by every node/">full node, ordering is decided by proof-of-work, and rewriting it costs more electricity than has ever been spent on the chain so far. There is no admin to bribe, subpoena, hack, or replace.
HOW_IT_WORKS
Mechanism
HOW IT WORKS
A block is a list of transactions plus an 80-byte header. The header contains: the previous block's hash (linking it to the chain), a merkle root that commits to every transaction inside, a timestamp, a difficulty target, and a 4-byte nonce. Miners assemble a candidate block, then double-SHA-256 the header over and over while varying the nonce until they find a hash below the target — this is proof of work. The winner broadcasts the block; every full node independently re-runs all the validation rules in microseconds and either accepts or rejects it. If two miners find a block at the same height, the network temporarily holds both, then converges on whichever side accumulates more proof-of-work first — the longest chain rule. To rewrite a confirmed transaction, an attacker would have to redo the proof-of-work for that block and every block after it, while the honest network keeps adding new ones ahead — practically impossible after ~6 confirmations.
1. A user signs a transaction with their private key and broadcasts it to the peer-to-peer network.
2. Nodes validate the transaction (correct signatures, unspent inputs, valid script) and gossip it to their peers; it sits in the mempool.
3. A miner picks transactions from the mempool, packages them into a candidate block, and builds the 80-byte header committing to the previous block hash and the merkle root.
4. The miner hashes the header trillions of times per second, varying the nonce (and extraNonce in the coinbase), until SHA256(SHA256(header)) is below the network target.
5. The miner broadcasts the winning block. Every full node re-verifies it independently — invalid blocks are dropped on the spot.
6. The next miner starts the loop again, using this block's hash as their "previous block hash". The chain has grown by one. After ~6 such blocks (~1 hour) the original transaction is treated as final.
WORKED_EXAMPLE
Three real consecutive blocks on the chain
EXAMPLE
BLOCK 839,999 ────────────────────────────────────────────────────────
hash 00000000000000000002a7c3fdbb37c1...
prev_hash 00000000000000000000ce6df95c39ba... ← points to block 839,998
merkle_root 39d6... (commits to 3,241 txs)
time 2024-04-19 23:53 UTC
txs 3,241
│
│ hash of block 839,999's header
▼
BLOCK 840,000 ──────────────────────────────── THE 2024 HALVING BLOCK
hash 0000000000000000000320284cfbecc6...
prev_hash 00000000000000000002a7c3fdbb37c1... ← matches block 839,999's hash
merkle_root abc1... (commits to 3,050 txs)
time 2024-04-20 00:09 UTC
subsidy 6.25 → 3.125 BTC (halved at this height)
txs 3,050
│
▼
BLOCK 840,001 ────────────────────────────────────────────────────────
hash 00000000000000000004b9c14f1e0a72...
prev_hash 0000000000000000000320284cfbecc6... ← matches block 840,000's hash
If anyone changes one byte in block 839,999:
→ its hash changes
→ block 840,000's prev_hash no longer matches
→ block 840,000 is invalid
→ every block after is invalid
→ attacker must redo all that proof-of-work faster than the honest network
→ infeasible. The chain is tamper-evident, not by policy but by mathematics.
Genesis block (block 0, 2009-01-03 18:15 UTC):
prev_hash 0000000000000000000000000000000000000000000000000000000000000000
coinbase msg "The Times 03/Jan/2009 Chancellor on brink of second bailout for banks"
KEY_PROPERTIES
APPEND-ONLY
Blocks can only be added at the tip. No edits, no deletes, no inserts — every node enforces this independently.
TAMPER-EVIDENT
Each block hash commits to the entire prior history. Change anything anywhere and every downstream hash breaks visibly.
REPLICATED
Every full node stores the whole chain (~700 GB in 2025). No master copy exists; nodes converge by following the most work.
COMMON_PITFALLS
Things that catch people out
PITFALLS
- "The blockchain is decentralised" is a property of the network of nodes, not of the data structure. A blockchain run by one company on one server has the same data structure but none of the security.
- Confirmations are probabilistic, not absolute. 0 confs is reversible; 1 conf is reorganisable; 6 confs is the convention for "settled"; exchanges sometimes require more for large deposits.
- A "51% attack" cannot create coins out of thin air — consensus rules still apply. It can only reorganise recent history and double-spend the attacker's own coins. It cannot steal someone else's funds.
- Block height is not time-10-minutes/">block time. Blocks target 10 minutes on average but vary; difficulty retargets every 2,016 blocks (~2 weeks) to keep the long-run pace. Short bursts of slow or fast blocks are normal.
- Bitcoin's blockchain is not a database. It is a write-mostly append-only log designed for verification, not query. Don't expect indexed lookups, schema evolution, or efficient state queries — that's what node-side indexes (Electrum, Esplora) exist for.
WHERE_YOU'LL_SEE_IT
Pages on this site that cover Blockchain in more depth:
RELATED_CONCEPTS
Other terms from Bitcoin Basics — click any to read its page:
TERMINOLOGY_INDEX
TERMINOLOGY
Blockchain
An append-only chain of blocks where each block commits to the previous one via its hash, making history tamper-evident.
Bitcoin
A peer-to-peer-p2p/">peer-to-peer digital cash system where coins are entries in a public ledger that anyone can verify and no one controls.
BTC
The ticker symbol for one bitcoin; one BTC equals 100,000,000 satoshis, the smallest divisible unit.
Satoshi (unit)
The smallest unit of bitcoin (1 sat = 0.00000001 BTC); all on-chain amounts are stored as integer satoshis to avoid floating-point errors.
Satoshi Nakamoto
The pseudonymous author of the 2008 Bitcoin whitepaper and the original Bitcoin software; their identity remains unknown.
Decentralization
No single party can change the rules or freeze funds, because thousands of independent nodes each validate every transaction themselves.
Peer-to-peer (P2P)
A network where every participant talks directly to other participants, with no central server in the middle.
Consensus
The shared agreement among nodes on which transactions are valid and which chain is the canonical history.