How exactly did the area code come to be?

In the mid-20th century, in response to the United States’ rapidly expanding telephone network, executives at the Bell System introduced a new way of dialing the phone. Until then, for the most part, it was human operators—mostly women—who had directed calls to their destinations. Dialing systems had reflected this reliance on the vocal cord. Phone numbers weren’t numbers; they were alphanumeric addresses, named after phone exchanges that encompassed particular geographic areas. The Elizabeth Taylor movie Butterfield 8 gets its name from that system: The Butterfield exchange served the tony establishments of Manhattan’s Upper East Side. Lucy and Ricky Ricardo, should you have attempted to call their apartment, were apparently reachable with a request for “Murray Hill 5-9975.”