Chain reorganization is a change to the currently accepted history of a blockchain in which one or more previously selected blocks are replaced by an alternative sequence.

Reorganizations can occur when different network participants temporarily build competing branches from the same common ancestor. Once additional information becomes available, nodes apply consensus rules and may switch to another branch.

Consequences

Transactions included in replaced blocks may become unconfirmed again and return to the mempool if they are not present in the new chain. Some transactions can also become invalid because the new history changes the state or ordering of events.

Small reorganizations can be a normal technical property of some networks. Deeper reorganizations can have greater implications for transaction finality and blockchain security.