If you're interested in how the shape of Connecticut's counties, including Hartford, have changed over time, we recommend the Atlas of Historical County Boundaries.
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When there's a risk of an area code running out of phone numbers, an 'Overlay Area Code' is created that has the same geographic boundaries as the existing area code. In this case, the 860 code has been Overlayed with the 959 area code. New phone numbers in the Bell Air area will be assigned with one of these codes: 860 or 959. As a result, placing a call in the Bell Air area will require 10-digit dialing (where you enter both the area code and then the phone number).