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What is a blockchain?

A shared, append-only ledger of transactions that many independent computers hold copies of and agree on, without any of them being in charge.

A blockchain records transactions in batches called blocks, each cryptographically linked to the one before it. Because every participant keeps a copy and the linking makes tampering detectable, changing a past entry means redoing the work that has been done since — which is what makes settled records expensive to alter.

Why it matters: the point is not the data structure, which is unremarkable, but that no single party needs to be trusted to maintain it. That is also the source of the trade-offs: agreement between thousands of independent machines is slower and more expensive than a single database, which is why throughput, fees and decentralisation are in permanent tension. See consensus and node.

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