Why do Midwestern and Rocky Mountain states share a boxy, sharp-edged shape while East Coast state borders look like the fever dream of an impressionist painter? Much of it has to do with when these states came into existence, and whether their borders were set by an 18th century land surveyor, a 19th century committee that wanted to balance the size of free states and slave states, or a 20th century government panel basing their decisions on aerial photography.

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