What Is a Reorg?
A reorg, reorganization, is the chain changing its mind about recent history. Two miners or proposers produce competing blocks. For a while both exist. Then one branch grows longer (or heavier, or more attested) and nodes discard the loser. Transactions that lived only on the discarded branch go back to the mempool, or vanish if they conflict with the winner.
How It Actually Works
- Short reorgs of one or two blocks happen. They are usually accidents of latency, not attacks. Deep reorgs are rare on large proof-of-work and proof-of-stake networks and become the news when they occur.
- A transaction can be confirmed, then un-confirmed, then confirmed again in a different block, sometimes with a different fee or not at all if a double-spend won.
- This is why confirmation counts exist, and why finality gadgets try to draw a line after which a reorg would require burning a fortune.
Risks and Common Mistakes
- Zero-conf acceptance on small coins, where hash rate is cheap to rent and a reorg is a business model.
- Panic during a one-block flip on Bitcoin or Ethereum. Check an explorer, wait a confirmation, then worry.
- Bridge deposits credited too fast. Bridges that treat soft confirmation as settlement have been the plot of more than one post-mortem. See bridges.
When It Matters
Taking payment, operating any service that credits deposits, and reading "chain split" headlines calmly. Most users will only meet reorgs as a delayed exchange credit.
Related Terms
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