Glossary
Hard fork
A hard fork is a rule change that old software rejects, which can split a blockchain into two.
If everyone upgrades, a hard fork is just an upgrade, like Ethereum's London or Merge. If part of the community refuses, two chains continue, like Bitcoin Cash in 2017 and Ethereum Classic in 2016.
See it in the timelines
What is a hard fork? With Bitcoin examples Bitcoin Cash hard fork (Aug 1, 2017) DAO hard fork and Ethereum Classic (Jul 20, 2016)
Updated
Sources
Facts written in our own words. Last checked Oct 14, 2026.