# Hard Fork

> A blockchain rule change that is not backwards compatible. Old software cannot follow the new chain.

Canonical URL: https://fudfomo.co/glossary/hard-fork
Source: What The Block! Dictionary v1.0 (last updated 2026-04-25), browsable at https://wtb.fudfomo.co.

## Definition

A hard fork changes a blockchain's rules in a way that older versions of the software cannot accept. Everyone running the network has to upgrade, or they end up on a different chain.

Most hard forks are coordinated upgrades that everyone agrees to. A contested hard fork can permanently split a network into two, each going its own way from the same shared history.

## Related terms

- [Fork](https://fudfomo.co/glossary/fork): A change to a blockchain's rules. Sometimes minor, sometimes big enough to split the network in two.
- [Soft Fork](https://fudfomo.co/glossary/soft-fork): A blockchain rule change that is backwards compatible. Old software still works, just with fewer features.
- [Consensus](https://fudfomo.co/glossary/consensus): The way a blockchain's computers agree on which transactions are valid and which version of the ledger is correct.

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