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?


1 Answer 1


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
    Commented Nov 28, 2016 at 16:09
  • What do you mean? The answer of BinaryMage mentions "is a change of the Bitcoin protocol".
    – Murch
    Commented Nov 28, 2016 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. Commented Nov 28, 2016 at 16:33
  • 1
    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
    Commented Oct 26, 2017 at 9:46

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