In cryptocurrencies, varied parties need to use common rules to maintain the history of the blockchain. A fork refers to a situation when a rule change occurs. ^[undefined] Forks have been used in cryptocurrencies in order to add new features to a blockchain or to reverse the effects of hacking or catostrophic bugs on a blockchain as was the case with the bitcoin fork on 6 August 2010 ^[undefined] or the fork between Ethereum and Ethereum Classic. Notably, blockchain forks have been widely discussed in the context of the bitcoin scalability problem.

Forks are related to blockchain splits.

As a result of a rule fork, a blockchain can split, i.e. diverge into two separate paths forward.

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Forks can be classified as soft forks or hard forks.

A hard fork is a rule change such that the software enforcing the old rules will see the blocks adhering to the new rules as invalid.

To prevent a blockchain split, all nodes running the old software shall upgrade to new rules.

^[undefined] Alternatively, all nodes using the new software shall return to the old rules as was the case of bitcoin split on 12 March 2013.

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Ethereum has hard-forked to "make whole" the investors in The DAO, which had been hacked by exploiting a vulnerability in its code. ^[undefined] In this case, the fork resulted in a split creating Ethereum and Ethereum Classic chains. In 2014 the Nxt community was asked to consider a hard fork that would have led to a rollback of the blockchain records to mitigate the effects of a theft of 50 million NXT from a major cryptocurrency exchange. The hard fork proposal was rejected, and some of the funds were recovered after negotiations and ransom payment. ^[undefined]

In contrast to a hard fork, a soft fork is a change of rules that creates blocks recognized as valid by the old software, i.e. it is backwards-compatible. ^[undefined] As for a hard fork, a soft fork can also split the blockchain when non-upgraded software creates blocks not considered valid by the new rules. ^[undefined]

A user-activated soft fork (UASF) is a controversial idea that explores how to perform a blockchain upgrade that is not supported by those who provide the network's hashing power.

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