Fork-Choice Rule
The algorithm nodes use to pick the canonical chain when multiple valid versions exist.
A fork-choice rule is the algorithm every node follows to decide which chain is the “real” one when competing valid branches exist. Bitcoin’s rule is simple: follow the chain with the most accumulated proof of work. Ethereum’s Proof of Stake uses a more elaborate rule (LMD-GHOST) that weighs validator attestations.
The fork-choice rule is what makes decentralized agreement possible without a coordinator — thousands of independent nodes, applying the same rule, converge on the same chain. It’s the subtle machinery beneath “the longest chain wins,” and its design directly affects a chain’s security against reorgs and attacks. When consensus researchers debate chain security, the fork-choice rule is often the crux.