What is a Fork?
A fork is a change to Bitcoin's protocol rules. A soft fork tightens rules and stays backward-compatible. A hard fork breaks compatibility and creates a permanent split. The most famous Bitcoin hard fork happened on August 1, 2017, when a group of miners and businesses ran software that increased the block size, creating Bitcoin Cash. The Bitcoin network kept running unchanged. Bitcoin Cash became a separate cryptocurrency.
Why It Matters
Forks are how Bitcoin evolves. They're also how disagreements get resolved when consensus can't be reached.
Soft forks are upgrades that tighten existing rules. Old software still sees new blocks as valid because the new rules are a subset of the old ones. SegWit (activated August 2017) is the most consequential example. Taproot (activated November 2021) is another. Soft forks happen when most miners and node operators agree to enforce a new rule.
Hard forks are breaking changes. Old software rejects blocks that follow new rules. Two networks. Bitcoin Cash forked from Bitcoin in 2017 over the block size debate. Bitcoin SV forked from Bitcoin Cash in 2018 over Craig Wright's disputed vision. Neither chain has ever come close to Bitcoin's price, hash rate, or developer activity, and a decade of attempted alternatives has shown that splitting off from Bitcoin almost always means trading network effects for a different ruleset that the market doesn't want.
The Block Size Wars (2015-2017) shaped the modern Bitcoin community. Big-block advocates wanted to increase capacity by enlarging blocks. Small-block advocates wanted to scale through layer-2 protocols like Lightning. The small-block side won the consensus battle, and the dissenters left to start Bitcoin Cash. The episode demonstrated that Bitcoin's governance, however messy, can resist concentrated pressure from miners and well-funded business interests.
How It Works
A soft fork adds restrictions that old software ignores. SegWit moved transaction signature data into a separate witness field. Old nodes didn't understand the new format but accepted SegWit blocks as valid because the legacy transaction structure still parsed. The upgrade reached activation when enough miners signaled support.
A hard fork removes restrictions or changes the rules in a way that old software can't accept. A block that exceeds the legacy size limit, or transactions that use a new signature scheme, get rejected by old nodes. If miners running new software produce blocks the old chain refuses, the chains diverge and never reconverge.
Most proposed protocol changes never reach activation. The Bitcoin community is slow and conservative by design. That's a feature, not a bug. Hard money requires hard-to-change rules.