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.

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.

This block could be called block B but could be represented as B_a. 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 B_{a. They are blocks} C_ab and C_ab2, sibling blocks from the same parent block. Only one can be added to the blockchain—so the network chooses C_ab. C_ab2 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 C_ab. This is block D_cab. C_{ab2 is the sibling of} D_cab's _{parent, so} C_ab2 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.

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