Hard Fork

A hard fork is a change to a blockchain's rules that is not backward-compatible: blocks valid under the new rules are rejected by software running the old ones, so every node and miner must upgrade to stay on the new chain. It is the blockchain equivalent of changing a language rather than adding a dialect — old and new speakers can no longer agree on what is valid.

Most hard forks are uncontroversial upgrades that the whole community adopts on schedule; Ethereum has executed dozens, from fee-market reform (London, 2021) to enabling staking withdrawals (Shanghai, 2023) and cheap rollup data (Dencun, 2024), each activating at a preset point with near-universal coordination. A hard fork becomes dramatic only when the community disagrees: nodes that refuse to upgrade keep extending the old rules, and the ledger permanently splits into two networks sharing history up to the fork. The classic examples are Bitcoin Cash splitting from Bitcoin in 2017 over block size, and Ethereum Classic continuing the original chain in 2016 after most of the community forked to reverse the DAO hack.

A common misconception is that hard forks are inherently hostile events; the overwhelming majority are routine, planned upgrades, and the contentious splits are rare exceptions. When a split does occur, holders end up with coins on both chains, which introduces practical hazards — replay attacks, where a transaction signed on one chain is rebroadcast on the other — that fork designers mitigate with replay protection.