What Is the ‘Longest Chain Rule’ and Why Is It Fundamental to Nakamoto Consensus?

The 'longest chain rule' is the principle that nodes in a Proof-of-Work network must always consider the chain with the most cumulative Proof-of-Work (i.e. the most difficulty or hash power spent) as the valid chain. It is fundamental to Nakamoto consensus because it is the mechanism that achieves decentralized agreement on the single, true transaction history, resolving conflicts and preventing double-spends in an adversarial environment.