Most people who talk about blockchain never look at one. They repeat the same line: each block contains the cryptographic hash of the previous block. It sounds right, but it is not quite true.
John D. Cook decided to look closer. He downloaded two consecutive Bitcoin blocks and inspected their raw bits. His post shows how the blocks actually connect, not in theory, but in hexadecimal reality.
He finds that what links one block to the next is not the hash of the whole block, but the hash of its header. The difference matters. The header is a compact summary of the block’s structure, holding the Merkle tree root that represents all transactions beneath it. Change a single transaction, and that root changes too, which changes the header and the hash. The entire chain reacts.
Cook takes readers line by line through what that header looks like. Magic numbers, byte order, proof of work, details that rarely enter the conversation outside engineering circles. He even shows the command line instructions that compute the double SHA256 hash, the exact sequence that ties Bitcoin together.
Along the way, he reveals how even trusted explanations are often half-truths. The zeros at the end of a hash, for example, are not decorative. They show the computational labor that earned the block its place in the chain.
Reading his breakdown, you remember that blockchains are not metaphors about trust or revolution. They are data structures built by people who value exactness. Beneath every slogan, there are bytes, hashes, and the quiet persistence of those who check the math for themselves.
It is refreshing to see a post like Cook’s at a time when technical precision often gets traded for storytelling. He reminds us that real understanding begins where the abstractions stop, at the level of the bits themselves.
Read John D. Cook’s original piece here: How blocks are chained in a blockchain