Written in August 2026 about events in 2024. This is a retrospective, not contemporaneous reporting.
What happened
Block 840,000 was mined at 00:09:27 UTC on 20 April 2024. At that height the block subsidy fell from 6.25 to 3.125 bitcoin, the fourth such reduction since the network started in 2009.
What it changed
The rate at which new bitcoin comes into existence, by exactly half, in one block.
The rule is four lines of arithmetic. Every 210,000 blocks the subsidy halves, starting at 50. At roughly ten minutes a block that is about four years. The series converges, and the sum of it is where the 21 million figure comes from: it is not a target anyone chose and maintains, it is what this sequence adds up to.
What makes it interesting is not the schedule but the enforcement. A miner who published a block claiming 6.25 after height 840,000 would have had that block rejected by every node running the rules, and the work spent on it wasted. Nobody prevents the attempt. The attempt simply does not produce a block anyone else will build on.
Compare that with how the supply of a national currency changes: a committee meets, weighs conditions, and decides. Both are policy. One is enforced by institutions and can be revisited; the other is enforced by every participant independently checking arithmetic, and revisiting it would require convincing all of them at once.
What it did not change
Miner revenue is subsidy plus fees, and only the subsidy halved. Fees are set by demand for block space and were unaffected by the event itself.
It did not guarantee anything about price. A scheduled, universally known supply change is the textbook case of information that market participants have had years to price, and treating each halving as a fresh catalyst assumes nobody was paying attention to a number published in 2009.
And it did not fix miner economics. Halving cuts revenue immediately while costs stay put, which squeezes the least efficient operators, and every cycle some of them shut down. The difficulty adjustment then does its work.
What we know now
Hashrate did not collapse. It kept setting records through the following two years, which suggests the industry priced the cut in advance rather than discovering it.
The most striking detail was on the day itself: the Runes protocol launched at the same block, and demand for that block's space drove transaction fees to a share of miner revenue far above normal, briefly making fees larger than the subsidy that had just been cut in half. It was a one-day artefact rather than a trend, but it was the clearest preview yet of the question that the schedule guarantees will eventually matter, when the subsidy has halved enough times to stop being what pays for security.