Chain Split

A chain split happens when Bitcoin's network diverges into two persistent, separately-valid chains - either accidentally (because of a software bug) or deliberately (because of a contentious hard fork).

Once split, each chain proceeds with its own blocks, its own miners, and its own followers. Coins that existed before the split duplicate: holders end up with one balance on each chain, controlled by the same private keys. A deliberate split is meant to be permanent; an accidental one can heal, as 2013 showed.

Notable chain splits in Bitcoin's history:

• Bitcoin Cash (BCH) - August 2017. The big one. The block size war of 2015-2017 culminated in BCH forking off Bitcoin on 1 August 2017 with 8 MB blocks and no SegWit. Today BCH still exists as a separate network with a few tenths of a percent of Bitcoin's hash rate.
• Bitcoin SV (BSV) - November 2018. A fork from BCH, not Bitcoin, with 128 MB blocks. Two forks removed from Bitcoin, and further still from relevance.
• March 2013 (accidental, and it healed). Version 0.8 replaced the database under the node software and, without anyone knowing, removed a limit that older versions had been enforcing. One large block split old nodes from new for 7 hours 41 minutes. Miners with most of the hash power then abandoned their own 25-block chain so that the chain older nodes could follow would overtake it. Unlike the two splits above, this one reconciled. The log of that night is in The 2013 Chain Fork.

What the BCH split clarified: when there's a contentious hard fork, the market decides which chain is "Bitcoin." Both technical and economic factors matter, but in practice the chain with the original ticker symbol, exchange listings, brand association, and developer mindshare retains the "Bitcoin" name and most of the value. The fork chain rebrands and trades as a separate asset, typically at a fraction of Bitcoin's value.

If a chain split were ever to happen again, holders of the pre-split chain would have coins on both sides, but the two chains' markets would price them very differently. This pattern is a meaningful structural defense against changes nobody really wants: the threat of "rebrand as an altcoin" keeps protocol changes conservative.