Because blockchains are distributed, two miners can sometimes find valid blocks at nearly the same time. This creates a fork: two competing versions of the chain tip. Nodes temporarily disagree about which block is 'latest.'

The network resolves this with a simple rule — the longest-chain rule (more precisely, the chain with the most accumulated proof-of-work). Whichever branch gains more blocks wins, and the losing branch is abandoned. Transactions in the abandoned branch are returned to the mempool and may be re-mined.

Soft fork: backward-compatible rule tightening — old nodes still accept new blocks (e.g., SegWit).
Hard fork: rule change that splits the network — old nodes reject new blocks (e.g., Bitcoin Cash split from Bitcoin in 2017).
Reorg depth: how many blocks are replaced when a fork resolves; deeper reorgs are rarer and riskier.

Event	Typical depth	Risk to merchant
1-block fork	1	Low if 1 confirmation
2-block reorg	2	Low if 3 confirmations
6+ block reorg	6+	Extremely rare on Bitcoin