SegWit
Also: segregated witness, BIP 141, BIP141
history · intermediate
Segregated Witness — a 2017 soft fork that moves signature data outside the legacy transaction body, fixing malleability and effectively increasing block capacity.
SegWit ("Segregated Witness") activated on Bitcoin mainnet at block 481,824 on 2017-08-24, ending the [block-size war](/glossary/block-size-war). It was a [soft fork](/glossary/soft-fork) — old nodes continued to validate post-SegWit blocks, just without seeing the witness portion.
What it actually did:
1. Moved signature ([witness](/glossary/witness)) data out of the transaction body. Pre-SegWit, signatures were part of the txid pre-image, so any party could re-sign a transaction differently and produce the same money flow with a different ID — the transaction malleability problem. Malleability made layer-2 protocols (like [Lightning](/glossary/lightning)) basically impossible to deploy safely. SegWit fixed it.
2. Introduced "block weight" in place of strict bytes. A block's weight limit is 4 million; witness bytes count for 1, non-witness bytes for 4. The effective block size rose from a hard 1 MB to a soft ~1.5–4 MB depending on how much of the block is witness data.
3. Defined new address formats (P2WPKH and P2WSH, encoded as bech32, prefix bc1q…) that are cheaper to spend, since witness data gets the discount.
SegWit also paved the way for Schnorr signatures and Taproot ([BIP 86](/glossary/bip86)), activated in 2021. Both presupposed a clean witness boundary.
For users: roughly 90 % of new transactions in 2026 are SegWit (native or wrapped). Wallets that haven't migrated are spending more in fees for no benefit.