The LOOP is an area of downtown Chicago, Illinois. It is the second largest commercial business district in the United States and is home to many of the city's major cultural institutions, such as the Art Institute of Chicago and the Chicago Symphony Orchestra. The LOOP is also known for its iconic architecture, including the Willis Tower, the Chicago Board of Trade Building, and the Chicago Cultural Center. The area is served by the Chicago Transit Authority's elevated train system, known as the 'L'.