SegWit (Segregated Witness)
SegWit (Segregated Witness) is a Bitcoin upgrade, active since August 2017, that moved signatures into a separate witness part of each transaction.
An upgrade active since August 2017 (block 481,824) that moved signatures into a separate part of the transaction. It fixed a long-standing bug and made room for more transactions per block. SegWit addresses start with bc1q.
What did SegWit fix?
Activated at block 481,824, it fixed transaction malleability: before it, anyone relaying a transaction could change its signature encoding and so its ID before it confirmed, which broke chains of unconfirmed transactions. With signatures outside the ID, the ID is fixed once signed. That made the Lightning Network possible.
Did SegWit make blocks bigger?
In effect. Blocks are now limited by weight, where witness data counts a quarter, so a block of SegWit transactions can hold more than the old 1 MB. SegWit addresses start with bc1q.
Sources
Updated 10 Oct 2026
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