Uncle Block (Ommer)
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Uncle block (also called ommer) was a valid block produced on the Ethereum network that wasn't included in the main chain, but was referenced by a canonical block and earned its miner a partial reward. Uncles existed only during Ethereum's Proof-of-Work era — they were eliminated by The Merge in September 2022.
How uncles worked
When two miners found a valid Ethereum block at the same height, only one could extend the main chain — the other became an uncle. Because Ethereum's block time was ~13 seconds (much shorter than Bitcoin's ~10 minutes), such collisions were common; without compensation, poorly-connected miners would be systematically disadvantaged.
The GHOST protocol (Greedy Heaviest Observed Subtree) solved this by allowing canonical blocks to reference recent uncles:
- The canonical block miner received a small bonus for including an uncle reference
- The uncle miner received a partial block reward (around 1.75 ETH before The Merge)
- Up to 2 uncles could be included per block
- Only uncles within 6 generations of the referencing block qualified
Transactions in the uncle block did not execute on the main chain — only the block header was acknowledged.
Uncle vs orphan
| Uncle | Orphan | |
|---|---|---|
| Valid PoW? | Yes | Yes |
| Parent in main chain? | Yes | Yes (for orphans in most chains) |
| Referenced by main chain? | Yes | No |
| Reward? | Partial | None |
In everyday language, uncles were Ethereum-specific; Bitcoin and most other chains have only orphan/stale blocks with no reward.
After The Merge
Proof-of-Stake replaced competitive mining with a single randomly-assigned block proposer per slot. Two validators can't produce competing blocks at the same height, so uncles no longer exist on Ethereum — post-Merge blocks always contain an empty ommer list.
