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Agreement (consensus) · 2 min read

What is a fork in blockchain?

By Oliver James Whitmore · Updated 20 October 2026

In one sentence

A fork is a split in a blockchain: either a short-lived branch when two valid blocks appear at once, or a change to the rules that some or all nodes adopt.

A temporary fork: C1 and C2 appear together; the branch that gets the next block wins.ABC1C2DstalelongerTransactions from the stale block go back to the mempool
A temporary fork: C1 and C2 appear together; the branch that gets the next block wins.

How it works

Temporary forks are a normal part of proof of work. Two miners may find valid blocks at almost the same moment, and for a short time different nodes see different tips. As soon as the next block is built on one of them, nodes switch to the longer branch; the other block becomes stale and its transactions go back to the mempool.

Soft forks change the rules by making them stricter. Blocks that follow the new rules are still valid under the old ones, so nodes that have not upgraded keep following the same chain. Bitcoin's SegWit upgrade, active from August 2017, and Taproot, active from November 2021, were soft forks.

Hard forks loosen or change the rules so that old software rejects new blocks. If everyone upgrades, the network simply continues under new rules; Ethereum schedules its upgrades this way. If a group refuses, two chains continue side by side. That happened on 1 August 2017, when Bitcoin Cash split from Bitcoin, and in July 2016, when Ethereum changed its history after the DAO incident and the unchanged chain carried on as Ethereum Classic.

"Forking" can also mean copying a project's source code to start a new network — a related but separate idea.

A simple example

Blocks A and B are shared by everyone. Two miners then produce C1 and C2 within seconds of each other. Half the network works on C1, half on C2. The next block found is built on C2, so the chain A–B–C2–D is now longer, every node adopts it, and C1 is left behind.

Common misconception

Misconception: Every hard fork creates a new coin.

In fact: A hard fork only produces two networks if some participants keep running the old rules. Planned upgrades that nearly everyone adopts change the rules without any split.

Where you meet it

  • Names of network upgrades and their activation heights.
  • News about chain splits.
  • Activation mechanisms such as BIP-9, in which miners mark their readiness for a soft fork.
  • Our side-by-side comparison of soft and hard forks.

Sources

This card explains a technical concept for general education. It is not a recommendation to acquire, sell or hold any crypto-asset, and it is not financial advice.