Free tools Windows power users keep installed
One-click scans. No signup required.
A Bitcoin hard fork changes consensus rules so that blocks invalid under the old rules can be valid under the new ones. Nodes that keep the old rules may reject those blocks. If the two groups continue following incompatible rules, the blockchain can split into lasting chains—but a rule change does not automatically create two enduring versions of Bitcoin.
How a Bitcoin hard fork works
Bitcoin nodes independently check whether blocks follow the network’s consensus rules. A hard fork changes those rules in a way that expands the set of blocks considered valid. BIP 99 defines it as “A consensus fork that makes previously invalid blocks valid.” (BIP 99)
As an Amazon Associate I earn from qualifying purchases.
- The rules change. Developers or other network participants propose or adopt a change to the conditions blocks must satisfy.
- Nodes may disagree. A node following the new rules may accept a block that a node following the old rules considers invalid. If participants do not converge on one rule set, the chains can diverge.
The Bitcoin developer glossary describes a hard fork as a permanent divergence that commonly occurs when non-upgraded nodes cannot validate blocks made under newer consensus rules. (Bitcoin developer glossary) BIP 99 says users need to upgrade to follow hard-fork rules. Whether a particular change results in a lasting split depends on how participants adopt and continue to enforce the competing rules.
Hard fork vs. soft fork vs. ordinary chain fork
| Type | Effect on block validity | Can old-rule nodes accept new-rule blocks? | Does it mean a lasting split? |
|---|---|---|---|
| Hard fork | Expands validity: something invalid under old rules becomes valid under new rules. (BIP 99) | Not necessarily; old-rule nodes may reject blocks accepted under the changed rules. (Bitcoin developer glossary) | No. It can lead to a persistent divergence if incompatible rules continue to be followed. (Bitcoin developer guide) |
| Soft fork | Contracts validity: some blocks allowed by the old rules become invalid under the new rules, while blocks already invalid remain invalid. (Bitcoin Core statement, 2016-01-07) | Old nodes can remain unaware of the new rules and still accept blocks that satisfy the old rules. (BIP 141) | Not by definition; it describes a rule-change approach, not an inevitable permanent split. |
| Ordinary chain fork or reorganization | No consensus-rule change is required; competing blocks can appear under the same rules. (Bitcoin developer glossary) | Nodes can follow the same rules while temporarily seeing competing chains. | Usually the competition can resolve as proof of work accumulates; “fork” here does not itself mean a hard fork. (Bitcoin developer guide) |
Why SegWit was a soft fork, not a hard fork
Segregated Witness (SegWit) is a useful counterexample to the idea that every major Bitcoin protocol change is a hard fork. BIP 141 specifies its deployment as a soft fork using version bits and explains that the witness structure could be introduced so older nodes remained unaware of the new rules. (BIP 141)
#1 Best Overall
Does a hard fork create a new Bitcoin?
Not necessarily. “Hard fork” identifies the kind of consensus-rule change; it does not say which chain participants recognize as Bitcoin or guarantee that a second chain will persist. A lasting split requires communities to keep following incompatible rules rather than converge on one chain.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What “fork” means depends on context
The word can also refer to a fork in software development: a separate line or copy of a codebase. That alone does not establish that Bitcoin’s consensus rules changed or that the network split. The Bitcoin developer glossary lists software forks separately from hard forks. (Bitcoin developer glossary)
Rank #2
Likewise, a temporary competition between blocks under unchanged rules is not the same thing as a hard fork. To understand a claim that Bitcoin has “forked,” check whether it describes a change to consensus validity, a short-lived chain reorganization, or only a separate software project.
Quick Recap
Rank #4
Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon at the time of purchase will apply.
