During 2015–2017, larger-block proposals, SegWit, miner- and user-led activation and hard forks competed; the outcome depended on software actually enforced by nodes and accepted by markets.
Growing demand filled blocks and raised fees, but the conflict was not only about transaction count. It also concerned node-operating cost, decentralization, the fee market, layered scaling and the legitimacy of rule changes.
The old consensus limited serialized block data to roughly one megabyte. Raising it adds on-chain capacity but also increases bandwidth, storage and validation load; no fixed value alone can satisfy unbounded long-run demand.
BIP 101 proposed an eight-megabyte hard fork followed by scheduled growth and miner-supermajority activation; Bitcoin XT shipped it in an alternative client. Critics disputed the parameters, transition safety and the claim that signalling could bind other nodes.
Developers publish code, miners select valid transactions and extend chains, and businesses choose supported services, but every full node accepts only blocks matching its rules. Hashrate cannot make an invalid block valid for a node that did not adopt the change.
BIP 141 SegWit separated witness data, fixed the principal transaction-malleability vector and introduced a limit of four million weight units. It was a soft fork: old nodes could accept blocks while upgraded nodes enforced stricter rules.
BIP 148 defined a user-activated soft fork that would reject non-SegWit-signalling blocks from 1 August. BIP 91 let miners coordinate earlier signalling at a lower threshold, so SegWit locked in before the risky date.
The New York Agreement paired rapid SegWit activation with a promised two-megabyte hard fork within six months. Major firms and miners signed, but not every node operator; signatures were therefore not themselves a consensus change.
Bitcoin Cash split on 1 August 2017 through its own hard fork, enlarged blocks and omitted SegWit. SegWit2x was separate: btc1 intended to double the base limit later on a chain it expected to be treated as Bitcoin.
SegWit activated at height 481,824. The SegWit2x hard fork planned for height 494,784 was suspended on 8 November for lack of consensus; extensive miner signalling did not prove universal acceptance of incompatible rules.
When studying the episode, separate proposal, implementation, signalling, lock-in, activation and enforcement. Check BIPs, client releases, block heights and node rules; slogans that either users won or miners decide omit essential technical and historical detail. BIP 101 — Increase maximum block size · BIP 141 — Segregated Witness · Bitcoin Core 0.13.1 — Segregated witness release · BIP 148 — Mandatory activation of SegWit · BIP 91 — Reduced threshold SegWit activation · Digital Currency Group — Bitcoin Scaling Agreement at Consensus 2017 · btc1 — SegWit2x implementation repository · Bitcoin ABC — 0.14.6 release
For the clearest picture, read this entry together with SegWit, Soft Fork, Hard Fork, Full Node, Bitcoin, Consensus rules. The reverse links also lead from Soft Fork, Hard Fork, Block weight, Gavin Andresen.