大螃嗨

Uncles are stale blocks i.e. with parents 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 (this is called the GHOST protocol). 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 are allowed per block.

http://www.ethdocs.org/en/latest/mining.html