Upgrades

Bitcoin changes its rules mostly by soft fork. A soft fork makes the rules stricter. Any block that follows the new rules is also valid under the old ones, so nodes that have not upgraded still accept the chain. They just do not enforce the new rule. An upgraded node enforces it, and so does everyone who relies on it.

How does a soft fork differ from a hard fork?

A soft fork only adds restrictions. Old nodes keep following the chain, because every new block still passes their checks. A hard fork changes the rules in a way old nodes reject. Nodes that have not upgraded then split off onto a different chain, so everyone has to upgrade together.

How does an upgrade activate?

Miners signal support in the version field of the block headers they produce. Each signal sets one bit, and the upgrade activates once enough blocks in a retarget period set it. SegWit needed 95% of the blocks in a period, or 1,916 of 2,016. Taproot needed 90%, or 1,815 of 2,016, and it also had a minimum activation height, so it could not take effect early.

Which upgrades are in this section?

• SegWit, or Segregated Witness, activated at block 481,824 on 24 August 2017. It fixed transaction malleability and added a new witness field and a new block weight limit.
• Taproot activated at block 709,632 on 14 November 2021. It added Schnorr signatures, a new output type and Tapscript.

Both upgrades were soft forks. Start with the SegWit page if you are new to them, because Taproot builds on the SegWit witness format.