Hard Fork

A hard fork is a protocol change so significant that it breaks compatibility with earlier software versions, splitting the network into two paths unless every participant upgrades together. Where a routine update tightens existing rules, a hard fork rewrites them: blocks or transactions the old code would have rejected suddenly become valid, so nodes still running the previous version can no longer follow the same chain as the upgraded ones.

In practice, developers propose the change, the community debates it, and a target block height or date is set for activation. Nodes that fail to update by then either fall off the network or, if enough hash rate stays loyal to the old rules, the two versions continue as separate chain splits, sharing history up to that point but diverging afterward, often with its own token.

Hard forks have shaped crypto history more than once. The 2016 hack of The DAO on Ethereum pushed developers to fork the chain and reverse the theft, a decision a minority rejected, choosing to keep mining the untouched ledger as Ethereum Classic. A year later, a dispute over Bitcoin's block size produced Bitcoin Cash, which itself split again in 2018 into Bitcoin Cash and Bitcoin SV. Not every hard fork spawns a rival coin: many pass with near-universal support and no lasting divide, since the goal is simply moving the whole network forward together.

The main risks are replay attacks, where the same signed transaction is valid on both resulting chains, and weaker mining security on whichever chain ends up with less hash power after the split.