Definition
The Heaviest Chain Rule is a fork-choice rule that selects the branch supported by the most observed work across an entire subtree of blocks, rather than only the single longest line of blocks. Its best-known formalization is GHOST (Greedy Heaviest-Observed Sub-Tree), proposed by Sompolinsky and Zohar in 2015 as an alternative to Bitcoin's simpler longest-chain selection.
How it differs from longest-chain
Under Bitcoin's longest-chain rule, blocks that lose a race are discarded and contribute nothing to security. GHOST instead lets those orphaned sibling blocks count as votes for their parent branch when resolving forks. Starting at the genesis block, the rule repeatedly descends toward whichever child roots the heaviest subtree. Because abandoned work still reinforces the chosen branch, networks can safely shorten block intervals and raise throughput without the orphan rate eroding security as quickly.
Where it is used
Variants of the heaviest-subtree idea appear in Ethereum's consensus, where a GHOST-derived rule (LMD-GHOST within the Gasper protocol) helps select the canonical head. Bitcoin itself deliberately keeps the simpler longest-chain rule, valuing conservatism and a long-tested security model over the throughput gains GHOST targets.
Contrast this with Bitcoin's approach in Longest Chain Rule, and see how fork choice fits into Nakamoto Consensus.
In Simple Terms
The Heaviest Chain Rule is a fork-choice rule that selects the branch supported by the most observed work across an entire subtree of blocks, rather…