-A **chain selection rule** is used to decide which chain is the "correct" chain. Bitcoin uses the "longest chain" rule, which means that whichever blockchain is the longest will be the one the rest of the nodes accept as valid and work with. For proof-of-work chains, the longest chain is determined by the chain's total cumulative proof-of-work difficulty. Ethereum used to use the longest chain rule too; however, now that Etheruem runs on proof-of-stake it adopted an updated fork-choice algorithm that measures the 'weight' of the chain. The weight is the accumulated sum of validator votes, weighted by validator staked-ether balances.
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