BIP91 & SegWit2x Lock-in, Activation

By Albert Szmigielski

There seems to be a lot of confusion about BIP91, SegWit, SegWit2x timelines lock-in and activation dates. This is a quick explainer.

BIP91 Forces signalling for BIP141
269/336 blocks are required to lock BIP91 in. 
Current BIP91 period started at block 476448. 
If at block 476783 we have 269 blocks that signaled bit 4, then bip91 will be locked-in. has a graphic showing BIP91 signaling.
Once that (lock-in) happens BIP91 will be activated 336 blocks later (roughly two and one third of a day). 
BIP91-enabled nodes will reject blocks that do not signal BIP141. 
BIP91 was devised to avoid BIP148 activation. 
Once signaling BIP141 becomes “mandatory” we still need to have enough votes for BIP141 to lock it in and activate it.
 Lock-in happens after 2016 blocks (roughly two weeks). 
Activation happens 2016 blocks later. 
Earliest SegWit lock-in could happen is Aug 10th.
As a result earliest SegWIt activation could be Aug 24th (2016 blocks after lock-in)

Originally published at on July 19, 2017.