Fork

A fork is a divergence in a blockchain — a point where one chain of blocks splits into two possible continuations, either momentarily by accident or permanently by a change in the rules. The word covers several related situations, which is why it confuses beginners.

Temporary forks happen naturally: two miners find valid blocks at nearly the same time, parts of the network briefly follow different tips, and consensus rules resolve the race within a block or two — the losing block is orphaned and its transactions return to the mempool. Protocol forks are deliberate rule changes and come in two kinds: a soft fork tightens the rules in a backward-compatible way, while a hard fork changes them incompatibly, requiring every node to upgrade. If part of the community refuses a hard fork, the chain splits permanently into two coexisting networks and currencies. The canonical example is 2017's split of Bitcoin and Bitcoin Cash over block size: holders of BTC at the fork block found themselves holding equal balances on both chains, which then diverged in price, rules, and community.

A separate usage — forking the code — means copying a project's open-source software to launch a new chain, as Litecoin did from Bitcoin's codebase; that creates a new network from scratch, not a split of an existing ledger. A common misconception is that every fork creates "free coins"; only contentious permanent chain splits do, and most forks are routine upgrades or momentary races that users never notice.