Fork (hard fork and soft fork)

A fork is a change to a blockchain's rules. A soft fork tightens the rules in a backward-compatible way: old nodes still accept the new blocks. Bitcoin's SegWit (2017) and Taproot (2021) upgrades were soft forks. A hard fork loosens or changes the rules so that old nodes reject new blocks, and everyone must upgrade to stay on the same chain.

If a community disagrees about a hard fork, the chain can split in two, each with its own coin and shared history up to the split. Bitcoin Cash split from Bitcoin on 1 August 2017 over block size. Ethereum Classic is the original Ethereum chain that continued after the 2016 hard fork reversing the DAO hack.

After a split, holders usually own coins on both chains. Beware of replay attacks and scam "fork wallets".