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A Bitcoin block is a data structure containing (in a simplified view) a header, along with the list of transactions confirmed within the block. Each block header has a specific set of fields that are listed in Table 3.4. In particular, a block header contains a pointer to the former block, effectively creating a blockchain.


As mentioned above, each block contains a pointer to the former block; the chain of blocks therefore constructs the blockchain (see Figure 3.5). The blockchain starts with a genesis block that has been generated by the creator of Bitcoin.

The blockchain is extended by appending blocks to the last block that has been added to the blockchain. The process of creating blocks is commonly referred to as mining, and it requires providing a proof-of-work that we detail in the next paragraphs.

Because different miners perform mining concurrently, it can happen that competing blocks are created at the same block height. This event is commonly

Table 3.4

Bitcoin Block Header Format





Block version number

4 bytes

Hash of previous block

Hash of previous block header

32 bytes

Merkle root hash

Transaction Merkle root hash

32 bytes


Unix time stamp

4 bytes


Current difficulty of the network

4 bytes


Allows miners to search a block

4 bytes

Chain of blocks starting from the genesis block forms the blockchain

Figure 3.5 Chain of blocks starting from the genesis block forms the blockchain.

referred to as forking and is depicted in Figure 3.6. Eventually, only one blockchain, the longest, can prevail. Blocks that are not part of the longest chain are therefore discarded and are referred to as orphan blocks.

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