longestchain

Longest chain is a concept used in many blockchain systems, especially those that rely on proof-of-work, to determine the canonical history of transactions. It refers to the chain with the greatest total amount of work or accumulated difficulty, rather than merely the most blocks. Nodes adopt the chain that has the most work and extend it by adding new blocks on its tip.

Operation and forks: Blocks build on a previous block, and sometimes multiple blocks are found at similar times, creating temporary forks. Nodes continue mining on the chain they see, and when a longer chain emerges, other nodes switch to it and discard blocks from the shorter fork. The discarded blocks become orphan blocks. The longest or heaviest chain thus becomes the agreed-upon record of transactions.

Terminology and weight: While the phrase longest chain is common, some discussions emphasize the heaviest or most-worked chain, since difficulty adjustments mean that simply counting blocks does not always reflect total effort. The key principle is the accumulated work, not block count alone.

Implications and limitations: The longest-chain rule provides probabilistic safety for consensus, but it does not guarantee immediate finality. Reorganizations can occur if a longer chain appears, though their likelihood decreases as more blocks are added. Security depends on the distribution of mining power; a majority of honest participants reduces reorg risk. In practice, transactions are considered increasingly secure after several confirmations.

See also: Nakamoto consensus, proof-of-work, blockchain, chain reorganization, orphan block.