The West Side is one of the three major sections of the city of Chicago, Illinois, U.S. The other two sections within Chicago that associate with the West Side are the North Side and the South Side. The West Side consists of communities that are of historical, cultural, and ideological importance to the history and development of Chicago. On the flag of Chicago, the West Side is represented by the central white stripe. The Chicago West Side has gone through many transitions in its ethnic and socioeconomic makeup due to its historic role as a gateway for immigrants and migrants as well as its role for funneling poorer African-American residents away from the wealthier lakeside neighborhoods and central business district. Today, the West Side consists of large mixed communities of middle class, working class, and low-income African American, Puerto Rican, and Mexican residents; some small communities of blue-collar, lower middle class and middle class white residents of historically Polish, Italian, Czech, Russian Jewish, and Greek, descent; and newer communities of middle-class, upper-middle class, and wealthy white residents created by gentrification. Major shifts continue to happen due to forces such as rapid gentrification, selective corporate investments, and unequal distribution of city resources. There are a range of services available on the West Side, especially educational, cultural, and medical institutions. The University of Illinois at Chicago is on the West Side, as is the United Center, home to the Chicago Bulls and Chicago Blackhawks. One of the nation's largest urban medical districts, the Illinois Medical District, is on the West Side. Three of Chicago's largest parks, along with much of the city's boulevard system, are in this part of the city: Humboldt Park, Garfield Park, and Douglass Park. The West Side is very accessible by the interstate and public transportation via the Chicago Transit Authority's many bus routes, the Chicago 'L', the Metra commuter rail, and the Eisenhower Expressway. Additionally, Cook County Jail, the United States' largest single site jail, and the Homan Square facility, maintained by the Chicago Police Department, are both on the West Side. Boundaries As with the other sides of the city, there is no consensus as to the exact boundaries of the West Side. The city's annexation of land beyond the original western border at Wood Street gave way to the development of the West Side. The city legislature added more land in 1869 through the annexation