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A Bitcoin hard fork is a change to consensus rules that makes blocks valid under the new rules even though the old rules would reject them. Nodes that have not upgraded can therefore disagree with upgraded nodes about which blocks are valid. If the groups keep following incompatible rules, the blockchain can split; the term alone does not guarantee that two lasting chains or coins will result.
How a Bitcoin hard fork works
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Participants change the consensus rules that determine which blocks the network accepts. Under a hard fork, the new rules expand the set of valid blocks: something rejected under the old rules can be accepted under the new ones. BIP 99 defines a hard fork as “A consensus fork that makes previously invalid blocks valid.” BIP 99
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Nodes running different rule sets may disagree about a block. An old-rule node can reject a block accepted by a node following the new rules. If participants do not converge on one rule set, their ledgers may diverge into separate chains. The Bitcoin developer glossary describes a hard fork as a permanent divergence that commonly occurs when non-upgraded nodes cannot validate blocks created under newer consensus rules. Bitcoin developer glossary: hard fork
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Users who want their nodes to follow the new rules need to upgrade. But a rule change that could cause a hard fork and a lasting chain split are not the same thing: the outcome depends on which rules participants follow and whether the divergence persists.
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Hard fork vs. soft fork vs. ordinary chain fork
| Type | What happens to block validity? | Can old-rule nodes accept new-rule blocks? | Does it mean a lasting split? |
|---|---|---|---|
| Hard fork | The new rules expand validity by allowing blocks the old rules rejected. BIP 99 | Not necessarily. Old-rule nodes may reject blocks accepted under the new rules. Bitcoin developer glossary | No. Incompatible chains can persist, but the term does not by itself establish that they will. |
| Soft fork | The new rules tighten validity: some blocks valid under the old rules become invalid, while blocks already invalid remain invalid. Bitcoin Core statement, January 7, 2016 | Older nodes can remain unaware of the new rules and may continue to accept blocks that comply with them. BIP 141 | Not necessarily; “soft fork” describes the rule change, not a guaranteed permanent split. |
| Ordinary chain fork or reorganization | No consensus-rule change is required; competing blocks can appear while nodes use the same rules. Bitcoin developer glossary | Nodes can follow the same rules while temporarily seeing different competing blocks. | It may resolve as proof of work accumulates rather than remain a lasting division. Bitcoin developer guide: forks |
Why SegWit is not a hard fork
Segregated Witness (SegWit) is a documented soft fork, not a hard fork. BIP 141 specifies its deployment using version bits and explains that the witness structure could be introduced through a soft fork while older nodes remained unaware of the new rules. BIP 141
Does a hard fork create a new Bitcoin?
Not automatically. A hard fork can result in a lasting chain split if groups continue using incompatible rules, but the phrase “hard fork” describes the rule change and possible disagreement—not which chain people recognize, whether both chains persist, or what either is called. A proposal may have the potential to cause a hard fork without producing an enduring split. Bitcoin developer guide: soft and hard forks
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What “fork” means depends on context
In Bitcoin discussions, “fork” can refer to a consensus-rule change, a temporary divergence between competing blocks, or a separate line of software development. A software fork—a copied or independently developed codebase—does not by itself show that Bitcoin’s network has split. The Bitcoin developer glossary treats these as distinct meanings. Bitcoin developer glossary: fork
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