Area Code Day

What the day is

Area Code Day is observed on November 10. It marks the North American Numbering Plan, devised by Bell System engineers in 1947, which divided the United States and Canada into numbered regions so that calls could eventually be connected without an operator.

Origin and background

The original assignment contains a piece of engineering logic that has outlived its reason. On a rotary dial, each digit takes time proportional to how far the dial travels, so 1 is quick and 0 is slowest. The engineers gave the shortest-to-dial codes to the areas expected to receive the most calls: New York City got 212, Los Angeles 213, Chicago 312, Detroit 313. Sparsely populated states received codes requiring far more dial rotation.

The middle digit also carried meaning. Codes with 1 in the middle indicated a state or region split into multiple areas, while 0 indicated a code covering an entire state. Touch-tone dialing and later exhaustion of available numbers destroyed both conventions.

Exhaustion is the modern story. Mobile phones, fax lines, pagers and second lines consumed numbers far faster than anticipated, and regions have been split or given overlays, where a new code covers the same geography as the old one. Overlays are why ten-digit dialing became mandatory in many places: two next-door neighbors can hold different area codes.

The geographic meaning has largely dissolved anyway. Mobile numbers travel with their owners, so an area code now often records where somebody once lived rather than where they are.

How to take part

• Look up your own area code's history. Most have been split or overlaid at least once.
• Notice the dial-time logic in the original assignments; it is a neat piece of constraint-driven design.
• Check whether your area requires ten-digit dialing, which catches out visitors constantly.
• Ask an older relative what their number was before area codes were required locally.
• Program full numbers with country codes into your phone, which saves trouble when traveling.