The 312 area code is a significant telephone area code in the United States, specifically serving downtown Chicago, Illinois. Here are three key facts about it:

Geographic Coverage: The 312 area code predominantly covers downtown Chicago, which includes the Chicago Loop and its immediate environs. This area is a central business district and one of the most iconic sections of Chicago, known for its high concentration of skyscrapers, cultural institutions, and historic sites​​​​.

Historical Significance: This area code was established in 1947, making it one of the original area codes created in Illinois. It reflects the rich history and development of telecommunications in the region. Over time, due to increasing demand for phone numbers, the 312 area code was split in 1996, leading to the creation of area code 847 for the northern suburbs of Chicago​​.

Surrounding and Overlay Area Codes: Area code 312 is completely surrounded by area code 773, which serves the rest of Chicago. Additionally, it is overlaid with area code 872, indicating the shared geographical region between these area codes. This overlay helps accommodate the growing need for new telephone numbers in the area​​​​.

In conclusion, the 312 area code is not just a dialing code for Chicago; it represents a vital part of the city's history and ongoing development. As telecommunications evolve, so does the story of area codes like 312, adapting to the ever-changing landscape of urban communication needs.