A fork occurs when enacting a new set of rules. A hard fork is a permanent change that would not allow for communication with prior bitcoins to occur. A hard fork is a permanent divergence in the blockchainA blockchain—the technology underlying bitcoin and other c... More protocol, not backwards compatible, meaning that nodes that have not upgraded can’t validate blocks created by upgraded nodes that follow newer consensusConsensus is a mechanism, usually used by most nodes on the ... More rules.