The Loop is what locals call downtown Chicago. It is the second largest downtown business district in the United States, after Midtown, Manhattan. Bounded on the west and north by the Chicago River, on the east by Lake Michigan, and on the south by railroad tracks, the lack of space created the necessity of height for buildings. From the Home Insurance Building, considered the first skyscraper, to the Sears Tower, the tallest in the United States, the Loop has had a long history of verticality in buildings.
The Loop got its name after the circuit that the "L" trains make around it.