A hard fork is a radical change to a blockchain's protocol that makes previously invalid blocks and transactions valid, or vice versa. Because the new rules are not backward-compatible, every node on the network must upgrade to the latest software version or be left on the old chain.
When a hard fork occurs, the blockchain splits into two distinct paths from the block where the change takes effect. If both chains retain active users and miners, two separate cryptocurrencies continue to exist. This is exactly what happened when Bitcoin Cash split from Bitcoin and when Ethereum Classic diverged from Ethereum.
Hard forks happen for several reasons:
- Feature upgrades — adding new functionality that requires changes incompatible with older software.
- Security fixes — patching critical vulnerabilities that cannot be addressed with a soft fork.
- Community disagreements — when stakeholders cannot agree on the project's direction, a contentious hard fork may result.
- Reversing transactions — in extreme cases, such as the 2016 DAO exploit on Ethereum, a hard fork can roll back malicious activity.
For token holders, a hard fork often means receiving an equal amount of the new token on the forked chain. However, this also introduces risks such as replay attacks, where a transaction on one chain is duplicated on the other.
Before investing in any project, it is worth reviewing its fork history and the strength of its governance processes to understand how protocol changes are managed.