Area code 773 went into effect in the city of Chicago, Illinois, on October 12, 1996, with a brief grace period that ended on January 11, 1997. The area code covers the city, excluding the downtown core.
Originally, the entire city and suburbs were in area code 312. Then, in 1989, area code 708 was created for the suburbs, leaving a rump 312. On August 10, 2007, the Illinois Commerce Commission announced that area code 872 will overlay area codes 312 and 773, and entered into service on November 7, 2009.