Blockchain Fundamentals
Hard Fork
A change to the protocol rules that is not backward-compatible: nodes that don't upgrade can no longer follow the new chain. If the community splits, the result can be two separate networks, as happened with Bitcoin and Bitcoin Cash in 2017.
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General education, not investment, legal or tax advice. Markets and regulation change quickly: where a definition depends on current law or on a recent event, check the latest coverage before relying on it.