Imagine you write a letter, seal it in an envelope, and stamp the envelope with wax. Then the next person stamps their envelope with an imprint of your wax seal — so anyone can tell, at a glance, whether your letter was ever opened. Now imagine a million people doing this simultaneously, each envelope stamped with the imprint of the one before it. That is, in essence, how a blockchain works.
In Plain English
A blockchain is a sequence of data containers called blocks. Each block holds a batch of transactions and a cryptographic fingerprint of the block that came before it. That fingerprint is called a hash. Because every block references the previous block's hash, the blocks form an unbroken chain. Change anything in an old block and its fingerprint changes — which immediately breaks every block that came after it.
You do not need to understand the underlying mathematics to use a blockchain wallet. But understanding the structure helps explain one of the most repeated claims in crypto: that blockchain records are permanent.
What Lives Inside a Block
Every block has two parts: a block header and a body.
The body is straightforward — it is a list of the transactions included in that block, from simple coin transfers to complex smart-contract calls.
The header is where the chain mechanics live. A Bitcoin block header is exactly 80 bytes and contains six fields:
- Version — signals which set of validation rules this block follows.
- Previous block header hash — the fingerprint of the block immediately before this one. This is what links blocks together.
- Merkle root hash — a single hash that summarises every transaction in the body. Any change to any transaction changes this value.
- Timestamp — when the block was mined, recorded in Unix epoch format.
- nBits (difficulty target) — a compact representation of how hard the puzzle miners had to solve.
- Nonce — a 4-byte arbitrary number miners adjust during mining (more on this below).
The Merkle root acts as a tamper-evident seal for the transaction list. If an attacker silently edits one payment buried in a block from three years ago, the Merkle root changes — and that change propagates outward, eventually breaking the chain.
What a Hash Is and Why It Matters
A hash function is a mathematical algorithm that takes any input — one byte or one gigabyte — and produces a fixed-length output. For SHA-256, that output is always 32 bytes (256 bits), expressed as a 64-character hexadecimal string.
Bitcoin uses double SHA-256: it runs SHA256 twice, SHA256(SHA256(block_header)). The result is the block's unique ID.
Three properties make hash functions powerful here:
- Deterministic — the same input always produces the same output.
- One-way — you cannot reverse a hash to recover the original data.
- Avalanche effect — change a single character in the input and the output looks completely different. There is no gradual drift; the output flips unpredictably.
The hash of a block is its fingerprint. It goes into the next block's header as the "previous block hash" field, permanently linking the two.
How the Chain Links Blocks Together
This is the mechanism that earns the name "blockchain." Each block header contains the hash of the block that came before it. Follow that chain of back-references from any block and you eventually reach block 0 — the genesis block, which has no previous hash because there is no block before it.
This structure means every block implicitly commits to the entire history that precedes it. To verify the chain is intact, you recompute each block's hash and check that it matches the "previous hash" pointer stored in the next block. If they agree all the way back to the genesis block, the chain is valid.
If even a single byte in an old block is changed, its hash changes. That breaks the pointer in the following block. That block's hash now changes too. And so on — a cascade of invalidity propagating forward to the present.
Why Tampering Breaks the Whole Chain
Spotting a tampered chain is easy. Fixing one is another matter.
To make a tampered chain valid again, an attacker must redo the proof of work for every block from the altered one up to the current tip. Proof of work is the process miners use to find a valid block: they repeatedly adjust the nonce (the 4-byte counter in the header) and hash the entire header until they find an output that falls below the network's difficulty target. Finding a valid nonce takes enormous computation; verifying that it is valid takes a fraction of a second.
Because thousands of nodes are simultaneously extending the honest chain, an attacker redoing old blocks would also have to outpace all honest miners combined. This is widely cited as requiring more than 50 % of total network hash rate — a prohibitive amount of computing power.
Each new block added on top of an old one is called a confirmation. Six confirmations is widely considered final for Bitcoin, because at that point the cost of re-doing the work vastly exceeds any plausible benefit.
This asymmetry — enormous cost to alter, trivial cost to verify — is the economic foundation of blockchain immutability.
What This Means for You as a User
You do not need to run the hash calculations yourself. Every wallet, node, and block explorer you interact with is already verifying the chain on your behalf.
When your transaction is included in a block and a few more blocks stack on top, you have a public, tamper-evident record. Anyone can audit the full history from the genesis block to today without needing to trust any single party. There is no edit button — only an append-only log.
The transactions locked into blocks are authorised by private keys and addresses. Understanding the block structure helps explain why a confirmed transaction cannot simply be reversed: undoing it would require rewriting every subsequent block across a global network of independent participants.
For long-term storage of significant amounts, many users take self-custody a step further with hardware wallets, which keep the keys that sign those transactions completely offline.
Key Takeaways
- A block has two parts: a body (transactions) and an 80-byte header containing six fields, including the previous block's hash.
- The Merkle root in the header seals all transactions; changing any payment changes the Merkle root and therefore the block's hash.
- Each block's hash is produced by double SHA-256 and stored in the next block's header — that back-reference is literally what makes a chain.
- Altering any historical block changes its hash, breaking every block that follows it in a visible cascade.
- Proof of work makes rewriting history prohibitively expensive; each confirmation adds another layer of computational protection.
