Blocks as ledger pages
A block is a batch of recent transactions plus metadata: timestamp, previous block hash, and a bundle of transactions waiting to be recorded. Once sealed, it becomes a permanent page in the chain—appended, not edited.
Think receipts stapled together. Each receipt lists who paid whom, the fees, and who assembled the page (miner/validator). After it’s stamped, the network moves on to the next page.
How blocks link
Each block’s hash depends on its own data and the hash of the previous block. Change one transaction and the block hash changes; the link to the next block snaps. This hash link is what makes blockchains tamper-loud.
Because every block points backward, editing history means redoing work (PoW) or re-staking/rewriting finalized slots (PoS) plus convincing everyone your altered chain is the real one. That cost and coordination make quiet edits impractical.
| Field | Example | Why it matters |
|---|---|---|
| Prev hash | 0000ab…c9 | Anchors to prior block |
| Tx root | f3b2…9e | Fingerprint of all txs |
| Timestamp | 2025-10-15 12:34 | Ordering/limits |
| Nonce / signature | 0000ef… (PoW) / validator sig (PoS) | Proof of work or stake |
| Block hash | 00009c…41 | Output that links forward |
What’s inside a block header
Fields vary by chain, but common pieces include:
Previous block hash: The anchor to the prior page.
Merkle/tx root: Fingerprint of all transactions in the block.
Timestamp: When the proposer claims to have built it (with rules/limits).
Nonce + difficulty: For PoW, proof you solved the puzzle at the set difficulty.
State root / receipts (on smart chains): Commitments to the post-transaction state and logs.
Transaction bodies sit alongside, carrying inputs, outputs, amounts, and signatures.
The Genesis block
The first block in any chain—Bitcoin’s Genesis block—has no parent hash. It sets the template for every block that follows. Coins from the Genesis block aren’t spendable; they anchor the chain but don’t circulate.
Every chain has its own Genesis story: Bitcoin’s includes a newspaper headline about bank bailouts; other chains embed launch data or initial allocations. All later blocks inherit trust from that starting point and the rules that follow.
Hashing and security
Blockchains lean on cryptographic hashes (e.g., SHA-256 on Bitcoin). Hashes are predictable in length but unpredictable in output; tiny input changes scramble the output. That makes tampering obvious and costly.
On PoW chains, miners must find a hash below a target—burning energy to prove effort. On PoS, validators stake capital and sign blocks, risking penalties for bad behavior. Either way, the hash links plus consensus rules make edits loud.
Why block size and time matter
Block size (or gas limits) sets how many transactions fit per block. Block time sets how often pages turn. Bigger blocks and faster times increase throughput but may affect decentralization and node costs. Chains tune these knobs differently.
Confirmations and reorgs
When your transaction lands in a block, it has one confirmation. Each new block on top adds another. Deep confirmations are harder to replace because an attacker must redo more work or stake. Occasional shallow “reorgs” can happen if two blocks arrive at once; the network converges on one chain tip, so waiting a few blocks reduces risk.
“Edit one page and every later hash screams. That’s the point.”