iTechGuides is reader-supported. When you buy through links on our site, we may earn an affiliate commission. As an Amazon Associate I earn from qualifying purchases. Learn more
A crypto hard fork is a change to a blockchain’s consensus rules that older validation software cannot necessarily accept. Blocks valid under the new rules may be invalid under the old ones, so node operators may need to upgrade. A lasting split into separate chains can happen if groups keep following different rules, but it is not an automatic result.
What a hard fork means
Blockchains rely on participating nodes applying shared rules to decide which blocks and transactions are valid. A hard fork changes those consensus rules in a way that is not backward compatible: a block allowed by the new rules may be rejected by software enforcing the old rules.
Bitcoin Core’s 2016 definition describes a hard fork as a consensus-rule change under which blocks invalid under the old rules may become valid under the new rules: Bitcoin Core’s statement on hard forks. The important point is the compatibility break between rule sets, not simply that developers changed or copied some code.
How a hard fork differs from other kinds of forks
| Term | What changes | What older software may do |
|---|---|---|
| Hard fork | Consensus rules change incompatibly. | It may reject blocks that follow the new rules. |
| Soft fork | Consensus rules change in a way designed to preserve compatibility for nodes that have not upgraded, provided blocks follow the new rules. | It can still validate blocks produced under the new rules. |
| Temporary block fork | Competing blocks appear at the same height while the network follows the same rules. | It is not necessarily caused by a rule change. |
| Software or Git fork | A separate development copy of a codebase is created. | It does not, by itself, change blockchain consensus or split the ledger. |
Ethereum.org also distinguishes hard forks from soft forks in its glossary. Bitcoin.org’s developer glossary explains the different meanings of “fork,” including competing blocks and codebase copies: Bitcoin Developer Glossary.
#1 Best Overall
What happens when a network adopts a hard fork
For a coordinated upgrade, developers release client software implementing the new rules and participants upgrade so they continue following the same consensus. A hard fork can also arise amid disagreement: if some participants keep validating old rules while others adopt new ones, the two groups may accept different blocks and follow separate chains.
That divergence does not guarantee two lasting chains or two valuable cryptocurrencies. One chain may lose participants and disappear quickly; separate chains may persist for some time or indefinitely. Bitcoin.org’s BIP 99 discusses these possible outcomes, and the NIST Blockchain Technology Overview (NISTIR 8202) explains that non-upgraded nodes may reject new blocks while old- and new-rule chains coexist.
Why node upgrades matter
A node checks blocks against the rules implemented in its software. If its operator does not upgrade when a hard fork activates, it may remain on the old rules and reject blocks accepted by upgraded nodes. The result depends on the network’s coordination and which rules the operator intends to follow; the word “hard fork” alone does not say which chain will prevail.
Recommended Free Tools
A dated example of activation
In an October 2016 FAQ, the Ethereum Foundation described a planned hard fork in which protocol changes were included in client software and activated at a particular block number. The FAQ said a node that did not upgrade could remain on the old-rule chain and be unable to operate on the post-fork network. This illustrates one way a hard fork can be coordinated; it is a historical explanation, not current Ethereum upgrade guidance. See the Ethereum Foundation’s 2016 FAQ.
Quick Recap
Best Value
What the term does—and does not—tell you
- It tells you that consensus rules changed incompatibly for old and new validation software.
- It indicates why upgrading may matter to node operators.
- It does not establish that a permanent chain split occurred, that both resulting chains have value, or that any particular exchange supports either chain.
- It is distinct from a short-lived competition between blocks under unchanged rules and from a software project being copied.
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.
