Final answer:
The Northwest Ordinance of 1787 is described, outlining the process for territories to become states and the early prohibition of slavery within new territories.
Step-by-step explanation:
The piece of legislation being described is the Northwest Ordinance of 1787, which established the precedent by which the United States would expand westward through the admission of new states rather than the expansion of existing states. According to the ordinance, territories could become states through a staged process of governance: a governor and council appointed by Congress with fewer than 5,000 white adult males; an elected legislature with 5,000 to 60,000 white male inhabitants; and the drafting of a constitution and petition for statehood when exceeding 60,000 inhabitants. Importantly, the ordinance provided for the protection of civil liberties including the prohibition of slavery in the new territories, a milestone in American history.