Fork: when a blockchain splits or changes rules

Soft Forks and Hard Forks

A fork is a change to a blockchain's protocol rules. The change is either a soft fork—backward-compatible, where old nodes still accept new blocks—or a hard fork, which breaks compatibility. Hard forks split the chain in two if part of the community refuses to upgrade: both versions continue producing blocks under different rules, with separate ledgers and separate communities.

The most famous hard fork is Bitcoin Cash, which split off from Bitcoin in 2017 over a disagreement about block size. Ethereum forked from Ethereum Classic after the DAO hack in 2016, with the majority of the community moving to the version that reversed the theft. In each case, holders of the original chain received an equal balance on the new chain by default, since the ledger up to the split was identical.

Soft forks are far more common as routine upgrade mechanisms. They let networks add features—SegWit and Taproot on Bitcoin, EIP-1559 and The Merge on Ethereum—without breaking the chain. Hard forks are rarer and more contentious because they require nearly unanimous community agreement to upgrade cleanly. When agreement falls short, the result is a fork war, two competing chains, and a public stress test of which one users, miners, and exchanges actually back.