What Was an Ommer (Uncle) Block in Cryptocurrency?

What Was an Ommer Block?

In blockchains that use proof-of-work, ommer blocks are rejected blocks created at the same time as another block. The network can only choose one block to add to the blockchain. In the Ethereum blockchain, this leftover block was called an ommer block. In other blockchains, they are called uncle or orphan blocks, referring to the familial relationships used to describe block positions within a blockchain.

Ommer blocks are no longer an issue under the proof-of-stake mechanism introduced to the Ethereum blockchain in 2022, but blockchains that use proof-of-work still produce these extra blocks.

Understanding Ommer Blocks

In a public blockchain like Ethereum and Bitcoin, it's essential to use a method that ensures data within the blockchain is verified and added by consensus. It is equally important to keep data from being changed. Many blockchains use a data structure called a Merkle tree to accomplish this.

A Merkle tree establishes ancestral relationships for blocks of data. Information from previous blocks is included in new blocks, similar to DNA passed on between generations. This creates the concept of a parent, parent's sibling, child, and sibling blocks similar to a graphical representation of a family tree.

A Family Tree Example

Here's how it worked—the first block in a tree could be named block A. The next block created from block A would be considered block A's child and would include A's information plus its own.

This block could be called block B but could be represented as B_a. B would be the name of the new block, and "a" referred to the data from the parent block. This parent/child relationship continued as more blocks were added with the information from each previous block. This created a family tree and blockchain.

Now consider if two blocks were validated and created simultaneously from B_a. They would become blocks C_ab and C_ab2, sibling blocks from the same parent block. Only one could be added to the blockchain—so the network might choose C_ab. C_ab2 was a fork of the original blockchain but was not added to it or validated. Finally, another block is mined on the blockchain that kept C_ab. This is block D_cab. C_ab2 was the sibling of D_cab's parent, so C_ab2 was an ommer block.

Special Considerations

These orphaned blocks were, in essence, bugs in the code—unintended and accidental byproducts of the mining process. However, Ethereum incentivized ommer block miners for several reasons:

• To allow for creating more ommer blocks as a byproduct of shorter block times and speed up the network.
• To decrease the centralization of incentives for large mining pools. These pools employed large mining farms and claimed the majority of the cryptocurrency rewards, leaving little for individual miners.
• To increase the network's security by supplementing the work on the main blockchain by allowing the work done on ommer blocks to be included.

Ommer blocks were purposefully incorporated into Ethereum's blockchain using the previous consensus mechanism's validation protocol, Casper the Friendly GHOST (Greedy Heaviest Observed SubTree). When a blockchain fork occurred from simultaneously created blocks, network validators selected which block was used.

Ethereum transitioned from proof-of-work to proof-of-stake consensus in 2022. Under the proof-of-stake consensus mechanism, ommer blocks ceased to be a concern because of the way blocks are created. However, Ethereum Classic, which forked from Ethereum after The DAO hack in 2016, still produces ommer blocks (although it calls them uncle blocks).

The Bottom Line

Ommer blocks were extra unaccepted blocks created by the Ethereum mining network under proof-of-work mining. Ethereum incentivized these blocks in addition to accepted blocks because there was work done to create them. After "The Merge" in 2022, Ethereum transitioned to proof-of-stake and no longer required mining—no more ommer blocks were produced.

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