Definition
Version bits are Bitcoin's standard way for miners to signal readiness for a soft fork by setting individual bits in a block's version field. Defined originally in BIP 9, the scheme lets multiple independent proposals be deployed in parallel, each assigned its own bit, so they can activate separately without colliding.
BIP 9 and its limits
Under BIP 9, a deployment has a start time and a timeout expressed as POSIX timestamps. The change locks in once a retarget period sees at least 95% of blocks signalling, otherwise it expires. The weakness is that activation depends entirely on near-unanimous hashrate cooperation, which lets a small minority of miners veto a change the wider community supports.
BIP 8 and forced activation
BIP 8 was proposed to address those flaws. It measures the start and timeout windows in block heights instead of timestamps, removing the manipulation risk from miners stalling the clock. More importantly it adds a lock-in-on-timeout (LOT) option: when set to true, miners must signal during the final period or risk producing invalid blocks, guaranteeing activation rather than leaving it to miner goodwill. This generalises the user-activated soft-fork idea from BIP 148.
Both schemes share the same on-chain signaling format, which is why activation methods like Speedy Trial reuse it. For how these mechanics played out in practice, see Speedy Trial and the general concept of a soft fork.
Full open-data reference: Bitcoin BIPs Reference — CSV / JSON + REST API, CC BY 4.0.
In Simple Terms
Version bits are Bitcoin’s standard way for miners to signal readiness for a soft fork by setting individual bits in a block’s version field. Defined…