Segregated Witness
Bitcoin's most contentious soft fork — fixed malleability, increased capacity, and unlocked Lightning.
Segregated Witness (SegWit) was proposed by Pieter Wuille in BIP 141 and activated on August 24, 2017 at block 481,824 after years of block size debate. It separated signature data (the witness) from transaction data, counting witness bytes at a 75% discount toward the block weight limit.
The upgrade fixed transaction malleability — a long-standing bug that prevented reliable payment channel construction. It also increased effective block capacity from 1 MB to roughly 2–4 MB without a hard fork, and introduced a new address format (bech32) starting with bc1.
Activation was anything but smooth. Miners delayed signaling for months while the UASF movement threatened to reject non-SegWit blocks. BIP 91 locked in at block 477,120, forcing miner capitulation. Big-blockers responded by forking to Bitcoin Cash six days before SegWit went live.
Key Benefits
Outcome
Activated successfully. Enabled the Lightning Network, reduced fees over time as adoption grew, and became a prerequisite for Taproot. Over 80% of transactions now use SegWit.
Key People
Key Blocks
Timeline
CoinWallet Spam Attack
Deliberate transaction flood clogs the mempool to push for bigger blocks.
UASF Movement Gains Momentum
Users threaten to reject non-SegWit blocks.
BIP 91 Locks In — SegWit Path Cleared
Miners signal overwhelming support for SegWit activation.
Bitcoin Cash Hard Fork
Contentious fork creates Bitcoin Cash at block 478,558.
SegWit Activates
Segregated Witness goes live at block 481,824.