What Is the “Longest Chain Rule” and Its Role in PoW Security?

The longest chain rule is the fundamental principle in Proof-of-Work that dictates which version of the blockchain is considered valid. In the event of a temporary fork or disagreement, nodes always accept the chain that has accumulated the most proof-of-work, meaning the chain with the most cumulative computational difficulty.

This rule ensures that all honest participants converge on a single, agreed-upon history and is the mechanism by which the network defends against malicious forks.