Forks: soft forks, hard forks and chain splits
How blockchains change their rules, and what happens when not everyone agrees.
A fork is a change to a blockchain's rules. A soft fork tightens the rules so old software still accepts the new blocks (SegWit in 2017, Taproot in 2021). A hard fork loosens or changes the rules so old software rejects the new blocks; if everyone upgrades there's still one chain, but if some refuse, the chain splits in two, as with Bitcoin Cash in 2017 and Ethereum Classic in 2016.
Soft forks: tighter rules, one chain
A soft fork makes something that used to be allowed no longer allowed. Blocks made under the new rules still look valid to old nodes, so they keep following the same chain, even if they don't understand everything in it. Bitcoin prefers soft forks for exactly this reason.
SegWit (Segregated Witness). Activated on August 24, 2017, at block 481,824. It moved signatures into a separate "witness" section, fixing transaction malleability (third parties could alter a transaction's ID before it confirmed) and effectively enlarging blocks by counting witness data at a discount. That made the Lightning Network practical.
Taproot. Activated November 14, 2021, at block 709,632, adding Schnorr signatures.
Hard forks: changed rules
A hard fork allows something old nodes consider invalid, such as bigger blocks or a different reward. Nodes that don't upgrade reject the new blocks and stay on the old chain. Ethereum ships its upgrades this way, on a planned date: London (with EIP-1559) in August 2021, the Merge to proof of stake in September 2022, Dencun in March 2024, Pectra in May 2025 and Fusaka in December 2025. Because nearly everyone upgrades, these don't split the network.
When a hard fork splits the chain
| Event | Date | Type | What happened |
|---|---|---|---|
| The DAO fork | July 20, 2016 (block 1,920,000) | Hard fork, split | After about 3.6 million ETH was drained from The DAO, most of the community moved the funds to a recovery contract. Those who refused to alter history kept the original chain: Ethereum Classic (ETC). |
| Bitcoin Cash | August 1, 2017 (split after block 478,558) | Hard fork, split | A group wanting bigger blocks instead of SegWit raised the block size limit to 8 MB. Everyone holding BTC at the split also got the same amount of BCH. See types of bitcoin. |
| SegWit | August 24, 2017 (block 481,824) | Soft fork | One chain; old nodes kept working. |
| Accidental split | March 11–12, 2013 | Unplanned | A database limit in older Bitcoin software made two versions disagree about one large block. Miners downgraded to rejoin a single chain within hours (documented in BIP 50). |
What a split means for your coins
- You hold the same amount on both chains, because the history before the split is shared. Your private keys control both.
- The two coins are separate assets with separate prices. Sending BCH to a BTC address, or vice versa, is a common way to lose money.
- Replay risk: right after a split, a transaction on one chain can sometimes be copied onto the other. Ethereum added chain IDs to signatures (EIP-155) after the ETH/ETC split for this reason.
- Scam alert
"Claim your free forked coins" sites that ask for your seed phrase are theft. Legitimate tools never need it typed into a website.
Who decides?
No single party. Developers propose changes, but each node operator decides which software to run, miners or validators decide which chain to build on, and exchanges and users decide which coin they value. A contentious change can split the community as well as the chain. Bitcoin's culture favors slow, backward-compatible soft forks; Ethereum's favors scheduled hard forks with broad coordination.
Limitations and risks
- Splits divide hashing power or stake, leaving the smaller chain easier to attack.
- Soft forks quietly downgrade the security of nodes that don't upgrade: they accept blocks without fully checking the new rules.
- Short "forks" happen naturally when two miners find a block at the same time; one becomes a stale block within minutes. That's normal, not a rule change.
Frequently asked questions
Is a hard fork good or bad for a coin's price?
Neither reliably. A planned upgrade usually passes quietly. A contentious split creates a second coin whose value is uncertain. This site doesn't predict prices.
Did I get Bitcoin Cash if I held bitcoin in 2017?
If you controlled your own keys at the split (August 1, 2017), yes, the same amount. If an exchange held your bitcoin, it depended on that exchange's policy.
Why is Ethereum Classic the "original" chain?
Because it kept the history without the DAO fork's change. The chain most people call Ethereum is the one that adopted the fork.
Was the Merge a hard fork?
Yes. It changed Ethereum's consensus rules from proof of work to proof of stake on September 15, 2022. Almost everyone upgraded, so there was one main chain afterward.
What is a stale or orphan block?
A valid block that lost a race: two miners found blocks at about the same time, and the network built on the other one. Its transactions usually go back to the mempool.
Where to next
Sources
- BIP 141: Segregated Witness
- BIP 50: March 2013 chain fork post-mortem
- Taproot topic, Bitcoin Optech
- Ethereum forks timeline (ethereum.org)
- EIP-155: Simple replay attack protection
- Bitcoin Cash, Bitcoin Wiki
Written for beginners and checked against the sources listed. Last reviewed November 4, 2026. This page is educational and is not financial, tax or legal advice. How we write and check pages.