Uncle Block

An uncle block is a valid block that lost the race to be included in the main chain but was still referenced and partially rewarded — a concept used by Ethereum during its proof-of-work era. When two miners solved blocks nearly simultaneously, only one could extend the canonical chain, but the loser could be cited by a later block as an "uncle," earning its miner a reduced reward instead of nothing.

Ethereum introduced this because its short block time — around 13 to 15 seconds, versus Bitcoin's ten minutes — meant simultaneous blocks were common. Discarding them entirely, as Bitcoin does with stale blocks, would have wasted a large fraction of honest mining work and disadvantaged smaller miners with slower network connections. For example, a miner whose block arrived a second too late might still have received a substantial portion of a normal block reward once another miner included it as an uncle, and the including miner earned a small bonus too. This design, related to the academic GHOST protocol, kept mining rewards fairer and factored uncles into chain-weight calculations.

A common misconception is that uncle blocks still exist on Ethereum; the 2022 merge to proof of stake eliminated mining and uncles entirely, since validators now take turns proposing blocks in fixed slots rather than racing. The term survives mainly in historical data, block explorers, and discussions of how proof-of-work chains handle fast block times.