What is an uncle block?
- Author
- CRYPTO PORT Editorial
- Published
- Updated
- Reading time
- 4 min
In short
An uncle block is a validly mined block that narrowly lost the race to the main chain but is still referenced by a later block and paid a partial reward. Ethereum used this while it ran on proof of work, to soften the disadvantage faced by smaller miners with slower propagation.
Key points
- A valid block that did not make it into the main chain
- A later block could reference it for a partial reward
- Designed to reduce the penalty on smaller miners
- Gone from Ethereum since the move to proof of stake
Definition
A validly mined block that was not included in the chain but can still be referenced by one of the next few blocks and earn a reduced reward. Also called an ommer block.
Ethereum's block interval was a matter of seconds, so propagation races were common. Discarding every losing block would have punished miners on poorer connections and pushed hash power toward large operators.
The protocol therefore let a block reference a recent losing block as an uncle. The uncle's miner received a reduced reward, and the referencing block earned a small bonus. The further back the uncle, the smaller its reward.
Unlike a main-chain block, an uncle's transactions are never executed; it counts only as proof that work was not wasted. After the 2022 move to proof of stake, where one proposer is chosen per slot, the mechanism became unnecessary.
Watch out for
- · Transactions inside an uncle are never executed, so those transfers did not settle
- · Naming and rules differ by chain; not every network has an equivalent
- · A high uncle rate can signal slow propagation or network trouble