The second 90 percent
The first 90 percent of the code accounts for the first 90 percent of the development time. The remaining 10 percent of the code accounts for the other 90 percent of the development time.
— Tom Cargill, Bell Labs, via the Ninety-ninety rule
This rule is so true it barely reads like a joke. Anyone who has shipped software has watched a project reach “almost done” and then stay there while the real work finally shows itself.
Not even AI can break it. It changes how quickly we can get through the first 90 percent: scaffolding, boilerplate, tests, happy paths, and enough of the shape of the thing that it feels almost done.
But the second 90 percent still requires human judgment. Someone has to know which rough edges matter, which tradeoffs are acceptable, and when the product is actually right.
