Reorg (Reorganization)

A reorg - chain reorganization - is what happens when a Bitcoin node learns about a chain with more accumulated work than the one it was following (loosely, a longer chain), and switches to it. The blocks it had at the previous tip become stale; the transactions they contained are returned to the mempool unless they're also present in the new chain.

The rule that drives reorgs is "longest valid chain wins" - more precisely, the chain with the most accumulated proof-of-work. When two miners find blocks at nearly the same height, the network temporarily splits. As soon as the next block is mined on one side, that side has more cumulative work, and nodes that were on the other side reorg to it.

Two flavors:

• Shallow reorgs (1-2 blocks deep). Routine. They happen dozens of times a year - the network saw between 28 and 93 stale blocks a year from 2022 through 2025 - and only affect transactions that were just barely confirmed. This is exactly why the 6-confirmation rule exists for high-value transfers - after six blocks, the probability of a reorg overturning your transaction is vanishingly small absent a massive coordinated attack (the whitepaper's own table gives an attacker with 10% of the hashrate about a 0.02% chance of catching up).
• Deep reorgs (3+ blocks). Rare and concerning. The deepest reorgs on Bitcoin's mainnet have come from software bugs that split the network: 53 blocks in August 2010, when the chain carrying the value overflow block was abandoned, and 25 blocks in March 2013, when version 0.8 nodes went back to the chain that older versions could follow. A deep reorg today would be a serious event worth investigating.

The economic implication: transaction finality on Bitcoin is probabilistic, not binary - a transaction is only as final as the work that has been mined on top of it. One confirmation is "probably fine for small amounts." Six confirmations is "fine for almost everything." Hundreds is "permanent for all practical purposes." See Double Spend for the attack that reorgs make harder, and Mining rabbit hole for the economics.

For the full story of the 25-block reorganization of March 2013, see The 2013 Chain Fork; for the 53-block one of August 2010, see the inflation bug postmortem.