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Not to be confused with Segregated Witness, which is commonly abbreviated as SegWit.

SegWit2x, (also called B2X or S2X), was a failed contentious hardfork that intended to double the block size limit. The attempt has been denounced as an attempt made by CEOs and owners of several Bitcoin businesses to introduce a change in Bitcoin with ulterior motives, including the replacement of the Bitcoin developers with Jeff Garzik.

Support rate

It failed to gain consensus among community:

  • over 80% of miners by hashare (though they were just signaling, and this has no cryptographic or economical meaning or repercussions)
  • just below 20% of users by economy (BitFinex futures market as BT2 token) (representing actual trades that were executed for money)
  • around 0% of Bitcoin developers (Bitcoin Core)

Attack or upgrade

It was created in a way that would trick users into unknowingly switching to B2X from BTC, due to:

  • no replay protection
  • simple SPV wallets without special tools to detect the problem would accept B2X blocks as BTC blocks
  • same address formats
  • code changes that allowed B2X nodes to pretend to be BTC nodes in order to connect and use P2P peers from BTC network

due to this qualities, many users considered B2X and attack on BTC network.

B2X was marketed by it's creators as an "upgrade".


The SegWit2x hardfork was declared cancelled in a joint statement by six individuals.[1] 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.[2] However, when the flag day arrived, it was found that an off-by-one error in the SegWit2x protocol made it impossible to solve the first required >1MB block, freezing the currency entirely.