what was an ommer block 
in blockchains that use proof of work ommer blocks are rejected blocks created at the same time as another block the network can only choose one block to add to the blockchain in the ethereum blockchain this leftover block was called an ommer block in other blockchains they are called uncle or orphan blocks referring to the familial relationships used to describe block positions within a blockchain ommer blocks are no longer an issue under the proof of stake mechanism introduced to the ethereum blockchain in 2022 but blockchains that use proof of work still produce these extra blocks 12understanding ommer blocksin 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 worked 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 ba b would be the name of the new block and a referred to the data from the parent block this parent child relationship continued as more blocks were added with the information from each previous block this created a family tree and blockchain now consider if two blocks were validated and created simultaneously from ba they would become blocks cab and cab2 sibling blocks from the same parent block only one could be added to the blockchain so the network might choose cab cab2 was a fork of the original blockchain but was not added to it or validated finally another block is mined on the blockchain that kept cab this is block dcab cab2 was the sibling of dcab s parent so cab2 was an ommer block investopedia sabrina jiangspecial considerationsthese 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 ommer blocks were purposefully incorporated into ethereum s blockchain using the previous consensus mechanism s validation protocol casper the friendly ghost greedy heaviest observed subtree when a blockchain fork occurred from simultaneously created blocks network validators selected which block was used 3ethereum transitioned from proof of work to proof of stake consensus in 2022 under the proof of stake consensus mechanism ommer blocks ceased to be a concern because of the way blocks are created however ethereum classic which forked from ethereum after the dao hack in 2016 still produces ommer blocks although it calls them uncle blocks 1