Uncle Block

An uncle block is an alternative valid block that was discarded or excluded from the main chain because the network selected a longer chain.

What Is an Uncle Block?

Most blockchains use a data structure called Merkle Trees to ensure the integrity of the transaction data within a block. This data structure organizes or links transaction data through ancestral relations, such as parents, children, siblings, and even so-called uncle blocks. 

An uncle block, sometimes referred to as an ommer block, is an otherwise valid block created at the same time as another block. However, it doesn’t make it to the canonical or main chain. It is instead discarded or omitted from the blockchain since the network can only choose one block to include.

The term uncle/ommer block specifically refers to this phenomenon on Ethereum, while the term “stale block” is used to refer to these kinds of blocks on the Bitcoin network. This phenomenon applied to Ethereum’s proof-of-work system, which only allowed one block to be added to the blockchain at a time. However, it no longer occurs on Ethereum since the network transitioned to a proof-of-stake algorithm in 2022. That being said, Ethereum Classic, which forked from Ethereum after the DAO hack, still generates uncle blocks.

How Do Ommer Blocks Occur?

Ommer blocks appear when two or more miners create valid blocks at nearly the same time. This is often due to the network nodes delaying, even if slightly, in accepting a block into the blockchain. Since block time in Ethereum PoW was 15 seconds, another miner could produce and broadcast another legitimate block almost immediately. 

To prevent the blockchain from splitting, the block that demonstrates the most mining difficulty or the best proof of work continues in the main chain while the rest become ommer blocks. Put differently, the network selects the longest chain – the chain that is accepted by the most nodes and continues to develop – as the right chain to add to the blockchain.

Because only one block can be added at a time, the other blocks, though valid, are removed from the chain. The transactions included in the ommer blocks may then be rolled back once the blocks are discarded. 

Despite being rejected, miners who produce the ommer blocks receive a portion of the block rewards along with the transaction fees, though these rewards are lower than those for the accepted block.