Fork

Most forks are accidents that last one block: two miners publish at the same height and the network picks a winner within seconds. A few are deliberate rule changes, and one of those arguments ended with a second chain splitting away at block 478,558 on August 1, 2017. When a headline says bitcoin forked, the first useful question is which of those things actually happened.

How it works

A fork is a branch, and bitcoin grows branches in three unrelated ways that share nothing but the name.

The first is a chain fork, and it is ordinary bad luck. A block takes roughly ten minutes to find and a second or two to reach the far side of the network, so now and then two miners solve the same height and each half of the network sees a different block first. Both are valid. Both build on the same parent. The tie breaks when the next block lands on one branch, and the loser becomes a stale block whose reward evaporates. Nobody votes, nobody negotiates, and the episode is usually over before you could refresh an explorer.

The second is a rule fork, which is what people mean when they argue about forks. The split here is not in the chain but in the software's definition of a valid block. Tighten the rules and old nodes still accept everything the new software produces: that is a soft fork, and bitcoin has done it repeatedly, from pay to script hash on April 1, 2012 to Taproot at block 709,632 on November 14, 2021. Loosen the rules and old nodes start rejecting blocks the new software considers legal: that is a hard fork, and it avoids a permanent split only if essentially everyone upgrades.

The third is a code fork, which touches consensus not at all. Anyone may copy the Bitcoin Core repository, change it, and publish the result. Bitcoin Knots is precisely that, a separate project maintained by a different developer that keeps the same consensus rules on purpose, so its blocks and Core's blocks are interchangeable. Thousands of repositories on GitHub are forks of bitcoin in this sense and almost none of them ever produce a block.

Only the middle kind can create a new asset, and only when the disagreement is deep enough that miners keep building both branches indefinitely.

Why this matters when you buy bitcoin

Forks reach an ordinary buyer as operational events at an exchange, not as protocol events.

Around any contested rule change, platforms freeze the parts of their business that assume a stable chain. Deposits and withdrawals stop, sometimes for days, while engineers wait for the situation to settle and decide whether a second asset now exists. That freeze is the part that costs you, because it tends to land exactly when the price is moving. If you keep coins on any of the exchanges we review and a disputed change has a known activation date, withdraw to a wallet you control well before that date rather than on it.

The second thing worth knowing is that a rule fork demands nothing from you. Every soft fork bitcoin has activated has left old wallets working. A twelve word seed phrase written down in 2016 still restores, still derives the same addresses, and still spends. Nobody has ever had to move coins to keep them.

The third is that "free coins from the fork" has been a scam script since 2017 and has never stopped being one. No genuine chain split requires your seed phrase, your private key, or a small deposit to unlock anything. Our guide to common scams covers the exact wording these messages use.

Three things a headline can mean by "fork"

Take three events that were all reported as forks and notice how little they have in common.

On March 11, 2013, a database limit made version 0.7 nodes reject a block that version 0.8 nodes accepted, and the chain ran as two branches for about six hours until developers asked pools to downgrade. That was an accidental hard fork caused by a bug, and it ended with everyone back on one chain and one asset.

On August 1, 2017, a group that wanted larger blocks began enforcing an incompatible rule set at block 478,558 and kept mining it. That was a deliberate hard fork. It produced Bitcoin Cash, a separate asset with its own price, its own miners, and its own history from that block onward, sharing every balance recorded before it.

Bitcoin Knots forking the Bitcoin Core codebase, meanwhile, split nothing whatsoever. It is the same consensus rules in different packaging, which is why a Knots node and a Core node sit on the same chain and agree on every block.

Fork vs stale block

A fork is the branch; a stale block is the remains of the branch that lost. Every accidental chain fork leaves exactly one stale block per losing branch, and that block is not invalid, only unlucky: it satisfied every consensus rule and simply arrived on the wrong side of a race. Explorers that display it will label it stale or, less accurately, orphaned, and the reward inside it was never spendable.

Fork vs chain reorganization

A fork is the moment two branches exist; a reorganization is what one node does about it. Your node may never observe the fork at all, if it happened to receive only the winning block. If it received the losing block first, it reorganizes: it disconnects that block, connects the branch with more work, and carries on. One is a property of the network, the other is an event in your own debug log.