BIP 141 — Segregated Witness, Consensus layer

BIP 141 (Segregated Witness, Consensus layer) is the consensus soft fork that moved signature (witness) data into a separate transaction structure, fixing third-party transaction malleability, introducing block weight (4,000,000 weight units) in place of the 1 MB serialized-size limit, discounting witness bytes, and creating a script-versioning path for future upgrades. It activated on 2017-08-24 at block 481,824. Repository status: Deployed.

Debate relevance: The pivot point of the 2015-2017 scaling era and the prerequisite for Lightning at scale; block-size, fee, and inscription debates all route through its weight accounting. Common misstatements: "SegWit removed signatures from blocks" — witness data is inside blocks and committed to via the witness commitment; pre-SegWit software simply does not parse it. "Blocks are still limited to 1 MB" — the limit is 4M weight units (roughly 2 MB typical, up to ~4 MB). "SegWit was a 4 MB blocksize increase" — only in the theoretical witness-heavy extreme.