Hard fork
A change to the consensus rules that allows previously invalid blocks. Old nodes reject such blocks; if not everyone goes along, two permanently separate chains emerge – and with them a new, separate currency.
Alsohardforkhard-forkchain splitsplitreplay protection
A typical example is raising the block size limit. The counterpart is the soft forkGlossarySoft forkA change to the consensus rules that only tightens them. Blocks that follow the new rules remain valid for old nodes – so Bitcoin can be developed further without a chain split, as with SegWit and Taproot.In the glossary →, which only tightens the rules.
Example: Bitcoin Cash
After the dispute over block size, Bitcoin Cash split off on 1 August 2017. From block 478,559 it followed its own rules, initially with 8 MB blocks and from 2018 with 32 MB. In November 2018, Bitcoin Cash split again – into Bitcoin Cash and Bitcoin SV.[1]
What a split means for you
- A new currency: the split-off chain has its own rules and its own price. Having ‘Bitcoin’ in the name doesn’t make it Bitcoin.
- A balance on both chains: if you hold your own keys at the time of the split, you own your balance on both chains afterwards – without doing anything. If your bitcoin are on an exchange, the exchange decides what it credits you.[2]
- Replay risk: without replay protection, a transaction you send on one chain can also be valid on the other. In that case, assume you are spending the coins on both chains.[2]
Related terms
These terms are closely connected.
- This termHard fork
- Soft forkA change to the consensus rules that only tightens them. Blocks that follow the new rules remain valid for old nodes – so Bitcoin can be developed further without a chain split, as with SegWit and Taproot.
- Consensus (consensus rules)The agreement of all nodes on which blocks and transactions are valid and which chain counts. It rests on fixed consensus rules that every node checks for itself – no vote, no boardroom.
- NodeA computer that runs Bitcoin software and exchanges transactions and blocks with other nodes. Together, the nodes form the Bitcoin network; full nodes check every rule themselves.
- AltcoinA collective term for all crypto-assets other than bitcoin. They differ greatly in purpose, technology and distribution of power – many have an issuer or were pre-allocated to founders and companies.
- Bitcoin CoreThe most widely used software for Bitcoin nodes, a direct descendant of Satoshi Nakamoto’s first program. It is open source, verifies all blocks and transactions itself and includes a wallet.
Explained in depth
These articles go into more detail:
- Deep dive · Stage 1Forks & controversiesSoft forks, hard forks, the block size war, Bitcoin Cash, OP_RETURN and BIP-110 – explained neutrally, with both sides and what to do if the chain splits.
- Deep dive · Stage 1HistoryFrom the 2008 whitepaper via Pizza Day, Mt. Gox and the block size war to US spot ETPs and the 2026 split: Bitcoin’s history, concise and sourced.
- Deep dive · Stage 2Bitcoin, not ‘crypto’What sets Bitcoin apart from other crypto assets – a launch without a presale, no issuer, a fixed monetary policy – and why we only explain Bitcoin.
More from „Technology“
Sources2 sources · 2 publishers
The superscript numbers in the text refer to these sources.
- Bitcoin Cash – Wikipedia (English) (accessed 28/09/2026)
- Navigating the BIP-110 (RDTS) Activation – Start9, 05.08.2026, updated 12.08.2026 (accessed 28/09/2026)
This entry is for education only and is not investment, tax or legal advice.