SegWit (Segregated Witness) is a Bitcoin protocol upgrade activated in August 2017. It changes how transaction data is structured: witness data (signatures and scripts) is “segregated” from the main transaction block and stored in a separate structure. This small structural change had major downstream effects — increasing effective block capacity, eliminating transaction malleability, and making the Lightning Network practical.
The Problem SegWit Solved
Transaction malleability was a long-standing Bitcoin bug: the transaction ID (txid) could be altered by a third party before confirmation by modifying signature data. This made building second-layer systems unreliable, since off-chain channels depended on referencing specific txids.
Block size limitation — Bitcoin’s 1MB block size limit was causing fee spikes and backlogs as usage grew. Increasing the limit required a hard fork (contentious). SegWit found a way to effectively increase capacity without a hard fork.
How SegWit Works
Traditional Bitcoin transactions include:
- Transaction inputs (which coins are being spent)
- Transaction outputs (where coins are going)
- Witness data (signatures proving authorization)
SegWit moves item #3 out of the main block into a separate witness data structure. Because the original 1MB limit applied to the main block only, this:
- Reduces the “weight” of each transaction in the main block
- Allows more transactions per block without changing the 1MB base limit
- Introduces a new block weight metric (max 4 million weight units) instead of raw bytes
Practical capacity increase: SegWit blocks can hold roughly 1.7–2MB of equivalent transaction data within the 4M weight unit limit, approximately doubling effective throughput.
SegWit Activation — The Blocksize War
SegWit was proposed in 2015 (BIP 141) but its activation became the center of the Bitcoin Blocksize War (2015–2017):
- Miners largely wanted a simple 2MB hard fork (bigger blocks)
- Bitcoin Core developers backed SegWit as a safer soft fork
- Bitcoin Cash faction ultimately forked off in August 2017 when SegWit activated, creating BCH
SegWit activated on Bitcoin via BIP 9 (miner signaling) on August 24, 2017, after a separate agreement (the “SegWit2x” compromise) arranged industry support.
SegWit activated on Litecoin first in May 2017, serving as a testbed before Bitcoin deployment.
Native SegWit vs. Wrapped SegWit
| Type | Address Format | Also Called |
|---|---|---|
| Wrapped SegWit | Starts with 3 (P2SH) |
P2SH-P2WPKH |
| Native SegWit | Starts with bc1q |
Bech32, P2WPKH |
| Taproot | Starts with bc1p |
P2TR |
Native SegWit (bech32) addresses offer better efficiency and are the modern standard. Legacy (starts with 1) addresses do not use SegWit.
SegWit and Lightning Network
The transaction malleability fix was the prerequisite for Lightning Network. Lightning channels work by creating pre-signed transaction chains — if any txid could be altered before confirmation, the entire channel structure could be invalidated. SegWit made these txids immutable, making Lightning viable.
Social Media Sentiment
SegWit remains a divisive topic in crypto history. Bitcoin supporters view it as an elegant engineering solution that avoided a dangerous hard fork. Bitcoin Cash proponents view it as an overly complex solution that prioritized developer ideology over simple block size increases. The Blocksize War produced the most vitriolic community split in Bitcoin’s history, resulting in multiple forks and lasting factionalism.
Last updated: 2026-04
Related Terms
Sources
- Lombrozo, E., Lau, J., & Wuille, P. (2015). BIP 141: Segregated Witness (Consensus Layer). Bitcoin Improvement Proposals.
- Bier, J. (2021). The Blocksize War: The Battle Over Who Controls Bitcoin’s Protocol Rules. Self-published.
- Poon, J., & Dryja, T. (2016). The Bitcoin Lightning Network: Scalable Off-Chain Instant Payments. Lightning Network Whitepaper.