A block is a file where certain data is stored. Blockchains are composed of blocks, with each valid block on a chain referring to its predecessor. What specific data is stored on a block, how blocks are validated and linked, how big blocks are, and so on can vary between blockchains. Common block components include: a block header, a block creation timestamp, the previous block’s hash, the nonce, the software version, the merkle root (a hash of all of this block‚Äôs transactions), the difficulty, and the number of transactions contained within the block.