What is Hard Fork?
Crypto Glossary Definition
A change in a blockchain protocol that is not backwards-compatible and requires all members involved to upgrade their software.
Why Hard Fork Matters
A hard fork is a protocol change that isn't backward-compatible with older software, meaning every node must upgrade or get left on a separate, incompatible chain. When the community disagrees over the change, this is how new coins are sometimes created (Bitcoin Cash splitting from Bitcoin is the best-known example).
Hard Fork in Practice
Suppose a blockchain community has spent months arguing over whether to raise a coin's block size limit to allow more transactions per block, and eventually the disagreement becomes irreconcilable, with one faction insisting on the change and another wanting to preserve the original rules. Developers favoring the change release new node software with the larger block size built in, and once enough miners and validators switch over to running it, the network splits: blocks that comply with the new, bigger size limit are rejected by anyone still running the old software, and vice versa, so the chain literally forks into two separate, incompatible ledgers going forward. Anyone holding coins on the original chain at the moment of the split now holds an equivalent balance on both resulting chains, since the transaction history up to that point is identical. This is essentially the well-known story behind Bitcoin Cash separating from Bitcoin, and it illustrates why hard forks are treated so seriously by node operators: participating requires an active choice to upgrade, and choosing wrong, or not choosing at all, can mean getting stranded on a chain the rest of the ecosystem has moved away from, unable to interact with exchanges or services that only support the new version.
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