Bitcoin doesn't have a central authority that can change its rules for everyone. Instead, participants choose which software to run and which rules to enforce. So what happens when someone wants to change those rules?
In Bitcoin, changes to the consensus rules can take the form of a soft fork or a hard fork. Despite their names, these aren't necessarily disagreements or attempts to create a new cryptocurrency. They're two different ways of changing the rules that determine which blocks and transactions the network considers valid.
TL;DR
A soft fork introduces stricter rules while remaining backward compatible. Blocks that follow the new rules can still be accepted by nodes enforcing the old rules, although those older nodes cannot fully verify the new restrictions.
A hard fork changes the rules in a way that isn't backward compatible. Blocks permitted under the new rules may be rejected by nodes following the old rules, meaning participants must upgrade to enforce and accept the changed rules.
Neither type of fork automatically splits the network. Whether the network actually splits depends on which rules participants adopt and whether competing versions continue operating.
What does it mean to fork Bitcoin?
Bitcoin's consensus rules determine what makes a transaction or block valid. Every fully validating node checks incoming blocks against the rules enforced by its software, rejecting anything that doesn't meet those requirements. A fork occurs when different versions of the rules or blockchain history diverge. In the context of software upgrades, the term refers to changes in the rules that nodes use to determine validity.
Not every software update is a fork. Developers regularly release improvements, bug fixes, and other changes that don't alter Bitcoin's consensus rules. A soft fork or hard fork specifically concerns changes that affect what the network considers valid.
What is a soft fork?
A soft fork changes Bitcoin's consensus rules by making them more restrictive. Under the new rules, certain blocks or transactions that were previously allowed may no longer be valid. Nodes enforcing the updated rules accept a narrower set of blocks than nodes following the older rules. This means older nodes can generally continue accepting blocks produced under the new rules, even though they don't enforce all the new restrictions themselves.
One example is Segregated Witness, commonly called SegWit, which activated on Bitcoin in 2017. It changed how certain transaction data is structured and validated while preserving compatibility with older nodes.
Soft forks allow Bitcoin to introduce new rules without requiring every participant to upgrade at the same time. However, successful activation still requires coordination, and older nodes don't receive the full validation benefits of the new rules.
What is a hard fork?
A hard fork changes the consensus rules in a way that breaks compatibility with the old rules. Under the new rules, something that older nodes would reject may now be considered valid.
A well-known example occurred in 2017, when disagreements over Bitcoin's block size limits contributed to the creation of Bitcoin Cash. The two networks continued separately, each following its own consensus rules and maintaining its own blockchain history after the split.
However, a hard fork doesn't always result in two lasting networks. If participants coordinate around the same updated rules, the network may continue without a persistent split.
Does a fork always split the network?
The word fork can make it sound as though Bitcoin automatically divides into two separate cryptocurrencies whenever its rules change. In reality, a rule change and a lasting network split are different things.
A soft fork can be adopted without creating a separate cryptocurrency. Older nodes may continue following the same chain, even though they don't enforce every rule introduced by the upgrade.
A hard fork creates the possibility of an incompatible chain because older nodes cannot accept blocks that violate their rules. If some participants continue supporting the original rules while others adopt the new ones, two networks can emerge and operate independently.
Why does this matter?
Bitcoin's consensus rules are what allow participants around the world to agree on a shared transaction history. Changing those rules affects how nodes determine which transactions and blocks they will accept.
Soft forks and hard forks offer different approaches to making those changes, each with its own tradeoffs. Soft forks preserve compatibility with older nodes but can leave those nodes unable to verify the new restrictions. Hard forks can permit changes that soft forks cannot, but require participants to adopt compatible software to remain on the updated network.
Understanding the difference helps explain why changes to Bitcoin are approached carefully. An upgrade isn't simply a matter of developers releasing new code. It also depends on the rules people are willing to enforce and whether they can continue participating in the same network.
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