Hard Fork
A hard fork is a non-backward-compatible change to the Bitcoin protocol that makes previously invalid blocks valid. All nodes must upgrade to follow the new rules, or the network splits into two separate chains with incompatible transaction histories.
How It Works
A hard fork loosens or changes the consensus rules so that blocks previously considered invalid become valid. Since old nodes reject these new blocks, the network splits: upgraded nodes follow one chain, and non-upgraded nodes follow another. Both chains share the same history up to the fork point but diverge afterward. Any bitcoin held at the time of the fork exists on both chains.
Bitcoin's development culture strongly avoids hard forks. The most prominent example of a contentious hard fork was Bitcoin Cash (BCH) in August 2017, which increased the block size limit. The BCH chain split away from Bitcoin, creating a separate network with separate value. This event demonstrated why hard forks are dangerous — they fragment the network, confuse users, and create security risks around replay attacks.
Bitcoin's preference for soft forks over hard forks is not just a technical choice — it is a philosophical one. Hard forks require coordinated action from every node operator, effectively forcing compliance. Soft forks allow the network to evolve without coercion. This conservative approach preserves Bitcoin's social contract: the rules you agreed to when you started running your node will not be changed out from under you without your consent.
Key Points
- Makes previously invalid blocks valid — non-backward-compatible
- All nodes must upgrade or the network splits into two separate chains
- Creates replay attack risks where transactions may be valid on both chains
- Bitcoin development culture strongly avoids hard forks
- Bitcoin Cash (2017) is the most notable example of a contentious hard fork