A fork (a branch, an offshoot) in a blockchain is a change to the rules according to which the network recognizes a block as genuine (valid). Since a blockchain is maintained by many independent nodes, any protocol update means that participants need to reach agreement on the new rules. Based on their compatibility with older versions, forks are divided into two main types.

Soft fork and hard fork

• Soft fork — a backward-compatible, soft change. The new rules are stricter, but nodes that have not updated their software can still interact with the updated ones. The network remains unified.
• Hard fork — a hard change in which the new rules are incompatible with the old ones. Nodes that have not adopted the update stop considering the blocks of the new nodes valid, and the network may split into two independent chains.

In the case of a hard fork, one of the resulting branches may eventually disappear, or it may survive — much depends on what share of computing power (hash rate) and community support each chain has. The branch with greater power has a better chance of sustained existence. Forks are a natural mechanism for the evolution of blockchain protocols, through which improvements, fixes, and fundamental changes to consensus rules are introduced.