Block 481,824 · 24 August 2017
SegWit activates
Segregated Witness, SegWit for short, changed how transactions are built without making older nodes obsolete. It moves the signatures, the witness data, out of the part of a transaction its identifier is computed from. That fixed a long-standing flaw, which let a third party alter the identifier of a transaction before it was confirmed, and it opened the way to protocols built on top of Bitcoin, such as the Lightning Network.
It also changed how blocks are measured. Instead of a size limit in bytes, blocks got a limit of four million weight units, in which witness data counts for less. The key figures of every poster since then show that weight next to the size.
Pieter Wuille presented the proposal at the end of 2015, and Bitcoin Core shipped it in October 2016, waiting for 95 % of the blocks of a two-week period to signal support. For months they did not, in what became known as the block size war, and activation came only after users and miners pushed it through in the summer of 2017. The rule was locked in at block 479,808, on 9 August 2017, and took effect two weeks later with block 481,824, mined on 24 August.
This block still carries the signal: the bit that voted for SegWit is set in its version field, which the poster prints as 0x20000002.