Abstract: At some point in the growth of successful economies, informal customary rules of contract, tort, and property are replaced by or supplemented by formal private laws. These transitions sometimes succeed and sometimes fail. Yet little research has examined why. What scholarship exists often asserts that success requires new formal private law rules to have evolved organically and incrementally from within a society. Japan’s successful private law transition at the end of the 19th century suggests otherwise. Japan’s transition was sudden and derived from exogenous legal traditions, but it was highly successful. Japan’s example suggests that what matters most to the success of a private law transition is how well the new private law rules integrate with preexisting business customs. True—organic, incremental change is more likely to integrate well, and so is more likely to succeed. But it is the harmony of integration that matters, not the source of the new rules. Figuring out these ingredients for success matters, because getting private law transitions right enormously impacts the well-being of persons throughout society.