Final answer:
The Northwest Ordinance of 1787 established the Ohio River as the boundary between free and slave states, prohibiting slavery north of the river in the Northwest Territory.
Step-by-step explanation:
Under the Northwest Ordinance of 1787, the boundary between free and slave states was established at the Ohio River. The ordinance prohibited slavery in the Northwest Territory, which included land north of the Ohio River, setting a precedent that shaped future debates over the extension of slavery in other territories. It was a significant piece of legislation that provided a process for the orderly admission of new states from the territory, established a geometric grid pattern for land survey, and included a Bill of Rights for settlers in these new lands.