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When people talk about possible changes to how Bitcoin works they sometimes say a particular change would require a hard fork. What does that mean? Can a hard fork cause problems?

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Simply put, a so-called hard fork is a change of the Bitcoin protocol that is not backwards-compatible; i.e., older client versions would not accept blocks created by the updated client, considering them invalid. Obviously, this can create a blockchain fork when nodes running the new version create a separate blockchain incompatible with the older software.

For potential future changes that would require a hard fork, see the associated wiki page.

  • Should 'hard fork' not be constrained to Bitcoin protocol, and strictly define a condition in a blockchain network where node consensus diverges permanently. – Goddard Nov 28 '16 at 16:09
  • What do you mean? The answer of BinaryMage mentions "is a change of the Bitcoin protocol". – Murch Nov 28 '16 at 16:17
  • He meant limiting the definition to Bitcoin is unnecessarily restrictive. Ethereum has had hard forks too, and @BinaryMage's description of what a hard fork is would work equally well there, except for the fact that Ethereum doesn't use the Bitcoin protocol. – Nik Bougalis Nov 28 '16 at 16:33
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    I think you mean HF is not forward-compatible. It is BWC in the sense that "the new understands the old" which is the common definition in most dictionaries. – jiggunjer Oct 26 '17 at 9:46

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