A fork is a change to a blockchain's protocol rules. A soft fork tightens rules (old nodes still accept new blocks). A hard fork loosens or changes rules (old nodes reject new blocks), potentially splitting the network into two separate chains if not all participants upgrade.
What Is a Fork (Hard vs. Soft)?
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The short version
A soft fork is like a road adding a speed camera, cars already following the speed limit don't notice. A hard fork is like converting a one-way street to two-way, everyone must update their maps or they'll drive into oncoming traffic. If some refuse to update, you get two parallel roads.
How It Works
Soft fork: new rules are a strict subset of old rules. Blocks valid under new rules are also valid under old rules, so non-upgraded nodes still follow the chain (they just don't enforce the new restrictions). Example: SegWit made certain old-style transactions invalid but non-upgraded nodes saw SegWit outputs as "anyone can spend" (valid under old rules). Hard fork: new rules are incompatible with old rules. Non-upgraded nodes reject new blocks, causing a permanent chain split unless everyone upgrades. Example: Ethereum's Constantinople hard fork added new opcodes, old nodes wouldn't recognize these as valid. If a community disagrees on a hard fork, both chains may persist (ETH/ETC split in 2016).
The 2017 Bitcoin Cash hard fork
In August 2017, part of the Bitcoin community disagreed about scaling strategy. They hard-forked at block 478,558, changing the max block size from 1 MB to 8 MB. Non-upgraded Bitcoin nodes rejected the larger blocks. Result: two permanent chains, BTC (original rules) and BCH (new rules). Anyone holding BTC at block 478,558 received equal BCH on the new chain. Exchanges had to list both. As of 2024, BTC is valued at ~$60,000 while BCH trades at ~$300.
What People Get Wrong
All hard forks create new coins
Only contentious hard forks (where both chains retain users) create persistent new coins. Planned upgrades where everyone upgrades (like Ethereum's regular hard forks) don't split, the old chain simply dies.
Soft forks are always minor changes
SegWit was a soft fork that fundamentally changed Bitcoin's transaction structure. Soft vs. hard describes backward compatibility, not importance.
Forks are dangerous and destabilizing
Planned, uncontentious forks are routine maintenance, Ethereum hard-forks roughly twice per year for upgrades. Only contentious forks that split communities are disruptive.
Keep Reading
Sources & Further Reading
- Bitcoin Hard Fork History
Community-maintained record of proposed and executed Bitcoin forks
- Ethereum Network Upgrades
Official timeline of all Ethereum hard forks and their changes
Questions People Also Ask
- Do I get free coins from a hard fork?
- If you hold coins before a contentious hard fork and both chains persist, you'll have coins on both chains. But forked coins often have low value and exchanges may not support them.
- How do I know if a fork is happening?
- Follow the official development channels of the blockchain you use. Major exchanges announce planned forks and any required user actions well in advance.
- Can a fork be reversed?
- A soft fork can theoretically be reverted by another soft fork. A contentious hard fork resulting in two living chains is effectively permanent, you can't force the other chain's users back.