Hard Fork

A hard fork changes a blockchain's rules in a way that older software considers invalid. After the fork block, nodes running the new rules and nodes running the old rules will accept different blocks and build different chains. If everyone upgrades, the result is a single upgraded network. If a significant group refuses, the chain permanently splits into two.

The most famous contested hard forks created new coins. In July 2016 Ethereum hard forked to reverse the theft from The DAO; those who rejected the reversal kept the original chain as Ethereum Classic. In August 2017 a dispute over block size split Bitcoin and produced Bitcoin Cash. Holders of the original coin received an equal balance on the new chain in both cases.

Planned hard forks are routine on many chains. Ethereum's Merge in 2022, Shanghai in 2023, Dencun in 2024, and Pectra in 2025 were all coordinated hard forks that the network adopted without a lasting split. The term sounds dramatic, but most hard forks are simply scheduled upgrades.