Crypto glossary

Hard Fork

A hard fork is a change to a blockchain's rules that is not backward-compatible: blocks valid under the new rules can be rejected by nodes that have not upgraded. If part of the network refuses the change, the chain can split into two.

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Why old nodes reject the new chain

A hard fork loosens or changes the rules, for example by allowing larger blocks, changing how fees work or altering account balances. Nodes running old software see the first block that uses the new rules as invalid and stop following that chain. To stay together, essentially everyone who validates the chain has to upgrade before the fork activates at an agreed block height or time.

Planned upgrades and contentious splits

Most hard forks are planned, coordinated upgrades with broad support. Ethereum, for instance, has introduced many changes through scheduled hard forks such as the London upgrade in August 2021, and the network continued as one chain.

When a significant group disagrees, two chains can continue side by side. In July 2016, after the DAO hack, Ethereum hard-forked to return the stolen funds; those who rejected the change kept the original chain running as Ethereum Classic. In August 2017, a dispute over block size led to Bitcoin Cash splitting from Bitcoin.

What a split means for holders

At the moment of a split, both chains share the same history, so anyone holding coins before the fork holds them on both chains afterwards. Whether the new coins have value depends on exchanges listing them and on demand. Exchanges and wallets decide individually whether to support the new chain and credit users.

Risks around hard forks

Replay attacks are a key danger: without replay protection, a transaction signed on one chain could be valid on the other, so spending coins on one chain could unintentionally move them on both. Exchanges often pause deposits and withdrawals around a fork. Scammers also use forks as a pretext, offering fake tools to claim new coins that ask for your seed phrase, which should never be entered anywhere outside your wallet.

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Frequently asked questions

Do I get free coins in a hard fork?

Only if the chain actually splits and the new chain survives. Even then, the new coins may have little value or support.

Is a hard fork dangerous for my coins?

Coins held with your own keys stay safe on the original history, but you should be careful with transactions right after a split and ignore unofficial claim tools.

Does every hard fork create a new cryptocurrency?

No. Most are coordinated upgrades where everyone moves to the new rules and one chain continues.

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All glossary terms · Educational reference only — not investment, legal, tax or financial advice.