Hard Fork
A rule change old software refuses to accept.
A hard fork loosens or changes a chain's rules so that blocks valid under the new software are rejected by software that has not upgraded. It is not backward compatible, which is why everyone has to move together.
If a meaningful group does not upgrade, the chain splits into two that each ignore the other. A soft fork is the opposite case: the rules tighten, so old software still accepts the new blocks.
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