Chain split

Most forks are planned upgrades that everyone adopts, or brief accidental branches that resolve within a block. A chain split happens when a hard fork is contentious and a meaningful part of users, miners or validators keeps running the old rules. From the fork block onward there are two independent networks with a shared history.

Well-known examples are Ethereum Classic, which kept the original chain after Ethereum reversed The DAO hack in 2016, and Bitcoin Cash, created in 2017 by supporters of larger blocks; Bitcoin Cash itself split again in 2018, producing Bitcoin SV. Holders at the fork block receive coins on both chains, and exchanges decide which ticker keeps the original name.

Splits bring practical risks: transactions can be replayed on both chains if the fork lacks replay protection, wallets and exchanges may pause deposits, and the minority chain often ends up with far less hashrate, making it vulnerable to 51% attacks. Unexpected splits can also come from software bugs when nodes running different versions disagree.