Reorganization

A reorganization, or reorg, occurs when a bitcoin node abandons part of its current blockchain in favor of a competing branch carrying more accumulated proof-of-work. Blocks that were part of the best chain become stale, and any transactions unique to them return to the memory pool. Most reorgs are a single block deep and arise naturally when two miners find blocks nearly simultaneously.

Why it matters

Reorgs are why bitcoin settlement is described as probabilistic rather than instant. A transaction in the latest block could still be displaced, but the probability of reversal falls steeply with each additional confirmation, which is the logic behind the convention of waiting six confirmations, roughly an hour, for high-value transfers. A deep deliberate reorg is the mechanism of a 51 percent attack, so the practical absence of such attacks on bitcoin is a direct measure of its security budget at work.

How it works

Nodes always follow the valid chain with the most total work. When a longer competing branch arrives, the node disconnects blocks back to the fork point and connects the new branch, updating balances accordingly. Deep reorgs on bitcoin have been rare and accidental: the most notable, in March 2013, ran about 24 blocks and was caused by a database inconsistency between software versions, resolved within hours by coordinated downgrade. Smaller proof-of-work networks with cheaper hashrate have suffered hostile reorgs, illustrating what bitcoin's scale protects against.