What Is a “Chain Reorg” and How Do Checkpoints Prevent It?

A chain reorganisation (reorg) occurs when a node discovers a longer, valid chain that replaces the current chain, causing a rollback of transactions. Reorgs are a natural part of PoW consensus but can be exploited in a 51% attack.

Checkpoints are pre-defined block hashes, often hardcoded into the protocol or released by core developers, that nodes are programmed to never revert past. By establishing a checkpoint, the chain's history up to that point is considered immutable, effectively preventing a malicious chain reorg that extends past the last checkpoint.