What is a hard fork? With Bitcoin examples

The idea

A hard fork is a rule change that is not backward compatible. Nodes that do not upgrade will reject blocks made under the new rules. If everyone upgrades, the network simply moves on. If some people refuse, the chain splits into two coins that share the same history up to the fork block.

A soft fork, by contrast, only tightens rules, so old nodes still accept the new blocks. SegWit (2017) and Taproot (2021) were soft forks.

Bitcoin Cash, August 1, 2017

The block size debate split Bitcoin. A group wanting bigger blocks ran a hard fork at block 478,558 on August 1, 2017, creating Bitcoin Cash with 8 MB blocks. Every BTC holder got the same amount of BCH. A few weeks later Bitcoin activated SegWit as a soft fork instead.

Other Bitcoin examples

Bitcoin Gold forked from Bitcoin in October 2017 to change the mining algorithm. In 2018 Bitcoin Cash itself split again into Bitcoin Cash and Bitcoin SV. Back in 2010, developers also fixed the value overflow bug that had created 184 billion BTC out of thin air; the fix required a quick chain reorganization.

Beyond Bitcoin

Ethereum's 2016 DAO fork created Ethereum Classic, and planned Ethereum upgrades such as London and the Merge were hard forks that nearly everyone adopted.