A blockchain , [1] [2] [3] originally block chain , [4] [5] is a growing list of records , called blocks , that are linked using cryptography. Note: If identical txids are found within the same block , there is a possibility that the merkle tree may collide with a block with some or all duplicates removed due to how unbalanced merkle trees are implemented duplicating the lone hash. Paper can be hard to counterfeit because of the complexity of physical seals or appearances. Each transaction spends the satoshis previously received in one or more earlier transactions, so the input of one transaction is the output of a previous transaction. Part Of. With blockchain, however, that job is left up to a network of computers. Although a fork is an actual divergence in block chains , changes to the consensus rules are often described by their potential to create either a hard or soft fork.