First seen rule

The first seen rule, is a rule that is part of the evaluation criteria of both blocks and transactions on the network.

First seen rule for Blocks

When evaluating blocks, a node considers the first seen block to be the first block that was broadcast, which it is able to completely validate and is the furthest from the genesis block in a valid chain of proof of work.

When two blocks are competing in an orphan race and a node is trying to build upon one of them, if a new block is discovered upon the competitor, the node will stop working on the block it saw first and move to the longest chain.

First seen rule for Transactions

When receiving transactions, the first seen rule is applied to determine which transaction is valid in the case of a double-spend. When a node detects a double-spend, it always considers the transaction that it received first as the valid spender of that coin.

The rule has been extended further to add, any blocks which are discovered that include the double-spend transaction, those blocks should also be considered invalid and the node should continue to mine against it, unless a second block is discovered on top of that block, indicating a majority of the network has determined that the other transaction was the first seen.