SegWit2x was a 2017 proposal for a hard fork that would have raised the Bitcoin block size limit from one megabyte to two megabytes. The intent was to raise the number of transactions that fit into each block and to shorten confirmation times. The proposal never activated.
A hard fork introduces rules that reject blocks the old software still accepts. Nodes that refuse the new rules form a separate chain. A soft fork introduces rules that reject only a subset of previously valid blocks. Old nodes that have not upgraded still accept every block the new rules permit. SegWit deployed as a soft fork in August 2017. SegWit2x scheduled a hard fork for mid-November 2017. The split never occurred.
The one megabyte block cap produced congestion whenever demand for space exceeded supply. Users attached higher fees to obtain priority. SegWit addressed the constraint without a hard fork. Developer Pieter Wuille published the SegWit specification in December 2015. The design relocated digital signatures from the transaction list into a new structure called the witness. The relocation freed space for additional transactions. The protocol replaced the byte count with a four-million-weight-unit limit. A block that carries only base data reaches the old one megabyte ceiling. A block that carries mostly witness data reaches roughly four megabytes. Average post-SegWit blocks settled between 1.1 and 1.4 megabytes.
SegWit2x emerged from a private meeting held in May 2017 at the Consensus conference in New York. Representatives from more than fifty companies and more than eighty percent of hash rate at that time signed the resulting statement. The agreement split into two steps. Step one activated SegWit. Step two, designated SegWit2x, would raise the base block size to two megabytes three months later. The proposal omitted replay protection. Critics warned that the absence of replay protection would expose users to accidental loss of funds on both chains.
Supporters of SegWit2x included large mining pools such as AntPool, BTC.TOP in addition to ViaBTC. Payment processors such as BitPay, Coinbase next to Xapo also endorsed the plan. Their public argument emphasized speed and cost; they claimed that a two megabyte base size would double throughput and would keep fees low. Opponents included Bitcoin Core contributors, node operators, users who viewed the hard fork as a corporate takeover. They argued that a two megabyte base size would raise the cost of running a full node – they also objected to the closed door nature of the New York meeting.
Developers released the SegWit2x code under the name btc1. Jeff Garzik served as lead maintainer. The repository tagged version 1.14.4 as the release candidate for the November fork. The code altered the block size rule, the difficulty adjustment algorithm, and the peer-to-peer protocol version – it omitted automatic replay protection. Users would have needed to add their own transaction markers to avoid replay attacks.
Community opposition grew during October 2017. A public letter signed by more than fifty Bitcoin companies and more than two hundred fifty developers denounced the hard fork. The letter cited lack of consensus, lack of replay protection, and rushed timeline. Hash rate support dropped from eighty percent to thirty percent within weeks. On 8 November 2017 the organizers suspended the fork. They posted a brief statement citing insufficient consensus.
After the suspension, the SegWit2x code base lost developer attention. The btc1 repository archived its issues and ceased releases. The episode reinforced the principle that protocol changes require broad agreement. SegWit continued to function. Lightning Network channels opened weeks later. Average transaction fees fell from a November peak of twenty dollars to below one dollar by January 2018.
SegWit2x differed from SegWit in scope and method. SegWit altered transaction structure through a soft fork. SegWit2x would have altered both transaction structure and block size through a hard fork. SegWit activated with miner and user consensus. SegWit2x failed to reach consensus. SegWit introduced weight units. SegWit2x would have retained weight units and would have doubled the base size. SegWit remains in production. SegWit2x never activated.
The legacy of SegWit2x includes a clearer process for future proposals. The episode demonstrated that hash rate alone does not dictate consensus. It also showed that replay protection is a prerequisite for any contentious hard fork. The Bitcoin network continues to process transactions under the SegWit rules. Block size now adapts to demand through the weight unit system. The two megabyte hard fork proposal remains a historical footnote.