BIP 9 — Version bits with timeout and delay

BIP 9 (Version bits with timeout and delay) defines a mechanism for deploying multiple soft forks in parallel using bits in the block version field. Miners signal readiness; if a threshold (95% in the original design) is reached within a retarget period before the timeout, the change locks in and later activates. It was used for the CSV-related soft forks (BIPs 68/112/113) and for SegWit. Repository status: Deployed.

Debate relevance: The reference point for any 'how do soft forks activate' exchange, and for the recurring claim that miners 'vote' on protocol changes. Common misstatements: "Miners vote on Bitcoin's rules via BIP 9" — signaling is a readiness-coordination mechanism, not a governance vote; nodes enforce the rules. Also, the 95% threshold was specific to the original design and has been varied in later deployments.