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In bitcoin related discussions people often say that something may require a "hard fork". What do they mean? What is a fork?

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Fork means in this context that devs are about to change some fundamental hardcoded rule. This may mean that the part of network which didn't upgraded client to new version will run on one set of rules and another upgraded part of network will run on other set of rules. In general, network fork means inconsistency of whole network.

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