What Is an Ommer (Uncle) Block in Cryptocurrency?

What Is an Ommer Block?

It's possible for two blocks to be created simultaneously by a network. When this happens, one block will be left out. This leftover block is called an ommer block. In the past, they were called uncle blocks, referring to the familial relationships used to describe block positions within a blockchain.

Key Takeaways

  • Ommer blocks are created in the Ethereum blockchain when two blocks are created and submitted to the ledger at roughly the same time. Only one can enter the ledger.
  • Ommer blocks are similar to Bitcoin orphans but have an integrated use, unlike their Bitcoin counterparts.
  • Ethereum miners or validators are rewarded for creating ommer blocks in the Ethereum system through transaction fees to pay for their work.

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.

Here's how it works—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.

Ethereum is transitioning from proof-of-work to proof-of-stake consensus. Under the proof-of-stake consensus mechanism, ommer blocks may still be produced and have transaction fees rewarded.

This block could be called block B but could be represented as Ba. B is the name of the new block, and "a" refers to the data from the parent block. This parent/child relationship continues as more blocks are added with the information from each previous block. This creates a family tree and blockchain.

Now consider if two blocks were validated and created simultaneously from Ba. They are blocks Cab and Cab2, sibling blocks from the same parent block. Only one can be added to the blockchain—so the network chooses Cab. Cab2 is a fork of the original blockchain but is not added to it or validated. Finally, another block is mined on the blockchain that kept Cab. This is block Dcab. Cab2 is the sibling of Dcab's parent, so Cab2 is 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 employ large mining farms and claim 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 are purposefully incorporated into Ethereum's blockchain using its consensus mechanism's validation protocol, Casper the Friendly GHOST (Greedy Heaviest Object Sub Tree). When a blockchain fork occurs from simultaneously created blocks, a two-thirds consensus rule from network validators selects which block is used.

Ommer Block

Investopedia / Sabrina Jiang

What Is an Ommer (Uncle) Block?

Uncle block is the old name for an ommer block. Developers and the Ethereum community decided that there was no reason to have gender-specific names, so they decided on ommer as the new name.

What Is Ethereum's Ommer (Uncle) Rate?

The ommer rate (previously the uncle rate) is the rate at which the network produces ommer blocks. The rate changes daily and depends on the number of transactions occurring.

What Is Ethereum's Ommer (Uncle) Reward?

Under the proof-of-work consensus mechanism, rewards for ommer blocks were a small percentage of the block reward, plus transaction fees. When Ethereum transitions to proof-of-stake, ommer blocks will receive transaction fees.

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  1. Ethereum Improvement Prosposals. "EIP-3675: Upgrade Consensus to Proof-of-Stake." Accessed Feb. 14, 2022.

  2. Ethereum.org. "Proof-of-Stake." Accessed Feb. 14, 2022.

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