What Are Consensus Rules?
Consensus rules are the fundamental laws that define valid Bitcoin transactions and blocks. Every full node enforces these rules independently. If a transaction or block violates any rule, it's rejected—period.
These rules are what make Bitcoin, Bitcoin. Change them, and you're running a different system. That's why consensus rules are extremely difficult to modify.
Core Consensus Rules
21 Million Limit
No more than 21 million bitcoin can ever exist. The block subsidy halves every 210,000 blocks until it reaches zero.
No Double Spending
Each UTXO (unspent output) can only be spent once. Once spent, it's gone.
Valid Signatures
Spending bitcoin requires a valid cryptographic signature from the owner's private key.
Inputs ≥ Outputs
A transaction cannot create bitcoin. Total outputs must be less than or equal to total inputs.
Valid Proof of Work
Block hash must be below the current difficulty target. No shortcuts.
Correct Block Subsidy
Coinbase transaction can only claim the correct subsidy + fees. Currently 3.125 BTC + fees.
Timestamp Sanity
Block timestamp must be greater than median of last 11 blocks and not more than 2 hours in future.
Block Size Limit
Blocks cannot exceed 4 million weight units (~4 MB max, ~2 MB typical).
What Happens When Rules Are Broken
When a miner produces an invalid block, here's what happens:
The invalid block (102*) and anything built on top of it is simply ignored. The miner who created it wasted their electricity—they get nothing. This is why miners have strong economic incentives to follow the rules.
Changing Consensus Rules
Consensus rules can be changed, but it's extraordinarily difficult. Any change requires:
| Change Type | Description | Risk Level |
|---|---|---|
| Soft Fork | Rules become stricter. Old nodes still accept new blocks. | Lower |
| Hard Fork | Rules become looser or change fundamentally. Old nodes reject new blocks. | Higher |
A hard fork that isn't universally accepted creates a chain split—two incompatible versions of Bitcoin. This is how Bitcoin Cash and other forks were created.
Consensus vs Policy
Don't confuse consensus rules with standardness/policy rules:
| Consensus Rules | Policy Rules | |
|---|---|---|
| Enforcement | Absolute—block rejected | Optional—tx not relayed |
| If violated | Never valid, ever | Can still be mined |
| Can be changed | Only by fork | Node operator's choice |
| Example | 21M limit | Minimum relay fee |
Policy rules (also called "standardness rules") are covered in the next section.
Frequently Asked Questions
Can consensus rules be changed?
Yes, but it's incredibly hard. Changing consensus rules requires overwhelming miner and node support (a "soft fork" or "hard fork"). The community has rejected many proposed changes because they would break Bitcoin's properties.
What happens if someone creates more than 21M coins?
Every node on the network rejects it. The block is invalid. Miners don't get the reward. It's mathematically impossible for more than 21M BTC to ever exist.
What's the difference between consensus and policy rules?
Consensus rules define valid blocks (must follow or get rejected). Policy rules are more lenient defaults—some nodes accept non-standard transactions, some don't. Policy is about relay and mining, not validity.