Fork
network · intermediate
A divergence in the chain or the rules. Either two valid blocks at the same height, or a change to the consensus rules.
"Fork" is overloaded. A *chain fork* happens momentarily whenever two miners find a block at nearly the same time; it resolves when the next block arrives. A *rule fork* is a change to what the network considers valid — split further into soft forks (tightened rules, backward-compatible) and hard forks (loosened or changed rules, incompatible).
Chain forks are routine. Two miners find blocks 7 seconds apart, peers see different blocks first, the network briefly splits, and the next block resolves it. Bitcoin handles these via "follow the heaviest chain" — whichever fork accumulates more proof-of-work wins, and losing blocks become orphans.
Rule forks are political. The 2017 SegWit2x rebellion attempted a hard fork to enlarge the block size; it failed. Bitcoin Cash succeeded as a separate chain because it had enough hash rate and developers to survive on its own. The cultural lesson Bitcoin took from those years was that hard forks are coercive — they force a choice on everyone — and that soft forks via broad consensus are the responsible default.