The rotary dial made every phone number a small act of physical accounting. To dial a digit you hooked a finger into the numbered hole, dragged the wheel around to the finger stop, and let go. The digit was never counted on the way out. It was counted on the walk back, as the spring pulled the dial home and a governor released a run of electrical pulses, one click for each number. A one cost a single click. A nine cost nine. A zero, sitting at the far end of the arc, cost a full ten.

Ten digits, and each of them priced at exactly its own click count, paid out in the slow return of the wheel. This arithmetic of pulses is easy to forget today, but it shaped the map of a continent. When the North American Numbering Plan was drawn up in 1947, its designers understood that some cities would be dialled far more often than others, and that on a rotary phone a short, low-numbered code was quite literally faster to spin. So the busiest places were handed the quickest codes. New York answered to 212, a compact string that ran off the dial in a hurry.

Nova Scotia, with far fewer calls to field, wound out the long way to 902, a code whose high digits meant more clicks, more travel, more time with a finger caught in the wheel. The plan looks arbitrary until you remember the mechanism underneath it. Every area code was a decision measured in pulses per digit, in how long the dial would take to unwind. The fastest spins went where the traffic was heaviest, and the geography of an entire phone network was quietly tuned to the physics of a returning wheel.