On this day in 1787, the Continental Congress passed the Northwest Ordinance, dividing up all the land west of Pennsylvania, north of the Ohio River, and east of the Mississippi River, into those little boxes you see when you fly over Illinois:
In 1781, Virginia began by ceding its extensive land claims to Congress, a move that made other states more comfortable in doing the same. In 1784, Thomas Jefferson first proposed a method of incorporating these western territories into the United States. His plan effectively turned the territories into colonies of the existing states. Ten new northwestern territories would select the constitution of an existing state and then wait until its population reached 20,000 to join the confederation as a full member. Congress, however, feared that the new states—10 in the Northwest as well as Kentucky, Tennessee and Vermont—would quickly gain enough power to outvote the old ones and never passed the measure.
Three years later, the Northwest Ordinance proposed that three to five new states be created from the Northwest Territory. Instead of adopting the legal constructs of an existing state, each territory would have an appointed governor and council. When the population reached 5,000, the residents could elect their own assembly, although the governor would retain absolute veto power.
The cadastral bits of the law explain why Chicago's streets form a grid and why Detroit has streets with evocative names like "13 Mile Road."