In Ethereum, what is an uncle?

Why does Ethereum take this approach, instead of having a single chain of blocks back to the genesis block?

If you mine a block, and it only gets included as an uncle, how much of the original reward do you get?

How long after the uncle is created is it allowed to be included into a block?

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    The motivation is to include stale blocks into the chain, so that the blockchain can have faster confirmation times. I haven't gone through it all, but the bulk of the reasoning for using "uncle" blocks seems to be laid out here. – morsecoder Sep 3 '15 at 17:31

An uncle is a block that in bitcoin would be considered an stale because its not on the longest chain (it's an alternative block at the same height as your parent).

Ethereum incentivizes miners to include a list of uncles when they mine a block. This has two main effects:

1) It decreases centralization incentive by still rewarding miners that produce stale blocks on account of them not being part of a big pool and hence hearing about blocks later (due to network propagation delays)

2) It increases the security of the chain by augmenting the amount of work on the main chain by that done in the uncles (so no work, or at least much less work, is wasted on stale blocks)

That being said, uncles introduce additional economic complexity that I'm not sure is well understood (like incentives to mine empty uncles)

Additionally, consider this paper here

(Grabbed from a Reddit post)

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Uncles are stale blocks, ie with parent that are ancestors (max 6 blocks back) of the including block. Valid uncles are rewarded in order to neutralise the effect of network lag on the dispersion of mining rewards, thereby increasing security. Uncles included in a block formed by the successful PoW miner receive 7/8 of the static block reward = 4.375 ether A maximum of 2 uncles allowed per block.

Source: https://github.com/ethereum/wiki/wiki/Mining#mining-rewards

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