From Wikipedia:

"The Chicago Loop is the historical center of Chicago's downtown. It is among the largest central business districts in the United States of America, after Midtown Manhattan.

The Loop is bounded on the west and north by the Chicago River. To the East, the Loop ends at Michigan Ave. although some also say it includes Grant Park all the way to Lake Michigan. The south boundary is Roosevelt Rd.

The value of its real-estate shaped an architectural style dominated by high-rises. Notable buildings include the Home Insurance Building, considered the first skyscraper; the Chicago Board of Trade Building, a National Historic Landmark; and the Sears Tower, the tallest in the United States.

Some of the historic buildings in this district were instrumental in the development of high-rises. Chicago's gridded or rational street numbering system originates in the Loop at the intersection of State Street and Madison Street."

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Chicago Loop - Wikipedia, the free encyclopedia