# What are Uncle Blocks?

> Uncle blocks are blocks that did not get mined onto the canonical chain. When two or more miners produce blocks at nearly the same time, uncle blocks are created.

> For the complete documentation index, see [llms.txt](/docs/llms.txt).

## What is an uncle block?

Imagine Ethereum as a worldwide group chat where everyone is trying to add the next message (block) at the same time. Only one message can become the official next one. Sometimes two miners create a valid block at almost the same moment.

Both blocks are valid.
Only one gets added to the official chain.
The other block becomes an uncle block.

## Why do uncle blocks happen?

Ethereum nodes are spread all over the world. When one miner finds a block, it takes a short time for that block to reach the rest of the network. During this short delay, another miner might also find a valid block.

Now the network has two valid candidates for the next block.
Eventually the network settles on one.
The block that is not chosen becomes an uncle.

## Why are uncle blocks rewarded?

The miner who created the uncle block still performed real work and produced a valid block. It arrived a little too late. Ethereum gives a partial reward to encourage decentralization and to avoid punishing miners who are physically far from large mining pools.

## Do uncle blocks matter?

Yes. Even though they do not become part of the main chain, uncle blocks help the network in several ways:

* They reduce wasted mining work
* They make mining more fair for smaller or slower miners
* They increase network security
* They give miners an incentive to include uncle references in future blocks

## Summary

An uncle block is a valid block that lost a timing race.
It was correct. It was a little too slow to reach the network.