What Is SegWit

Segregated Witness — SegWit — activated on Bitcoin in August 2017, following years of debate and development. It was a soft fork: backwards compatible, gradually adopted, no chain split.

What SegWit actually changed:

Transaction structure. Before SegWit, every transaction included the digital signature data — the witness — inside the transaction itself. SegWit moved the witness data to a separate, extended part of the transaction. This reduced the effective size of most transactions, fitting more into each block.

Transaction malleability fixed. Before SegWit, a specific attack existed called transaction malleability — the ability to alter a transaction’s ID without invalidating it. This was a subtle bug with significant implications for second-layer protocols. SegWit fixed it by separating the witness data that was being manipulated.

Lightning Network enabled. The fix for transaction malleability was a prerequisite for the Lightning Network — Bitcoin’s second-layer payment system. Without SegWit, building a reliable Lightning Network was not possible. SegWit’s activation in 2017 directly enabled Lightning development to accelerate.

Why SegWit succeeded where larger block proposals failed: it was a soft fork that maintained backwards compatibility, addressed multiple technical problems simultaneously, and gave users a choice to adopt it gradually rather than forcing an immediate split.

SegWit adoption grew slowly — wallet and exchange support took years to become standard. But it represents exactly how Bitcoin governance is supposed to work: careful development, community debate, soft fork activation, gradual rollout, no coercion.

Tomorrow: a story — running a node for the first time. What it actually felt like.

— The Daily Bit