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Not to be confused with SegWit.

SegWit2x, (abbreviated B2X or S2X, and originally called SegWit2Mb), was a failed contentious hardfork attempt outlined in the New York Agreement that intended to double the block size limit. The hardfork has been denounced as an attempt made by CEOs and owners of large Bitcoin businesses to introduce changes to the currency's protocol and development cycle with ulterior motives.[1]

Though over 80% of miners signaled intention for SegWit2x and the New York Agreement, it failed to gain any consensus among the community and Core developers. As it became clear that the execution of the fork would lead to a currency split, the address format was deliberately kept identical and no replay protection was implemented, which would have caused many BTC users to inadvertently use B2X. Additionally, code changes were made that allowed B2X nodes to pretend to be BTC nodes in order to connect and use P2P peers from the bitcoin network when synchronizing. Due to these qualities, many users considered B2X to be an outright attack on the bitcoin network, but its designers continued marketing it as an upgrade.

Cancellation and aftermath

The SegWit2x hardfork was declared cancelled in a joint statement by six individuals.[2] In the following hours, the price of B2X futures (called BT2 on Bitfinex) dropped to 1% of the price of Bitcoin (BTC, called BT1 as a Bitfinex future).

Despite the cancellation, some parties intended to proceed with the hard fork.[3] However, when the SegWit2x nodes split from the rest of the network on November 17, it was found that they had done so one block earlier than anticipated, at block 494782.[4] This was due to an off-by-one error as a result of counting up from the genesis block instead of from block 1. After splitting off, SegWit2x nodes were then waiting for the first block of >1MB to begin the chain fork. But the protocol rules say that the block size increase does not occur until block 494784, so because block 494783 is both required to be at most 1 MB by the original rules and greater than 1 MB by the new rules,[citation needed] SegWit2x had come to a permanent standstill.[4]