More and more commercial phone numbers are being advertised with a name or word as part of the number. We are urged to dial 335-DIET or 970-LOAN. This is a small historical regression, requiring the use of letters that the phone company made obsolete decades ago.

Where did the old alphanumeric dial plate come from? Most of the world never used letters. And where did it go? The story begins in the telephone’s infancy.

At first, central-office operators sat at switchboards, completing connections in response to spoken requests like “Ring Dr. Smith, please.” There were few enough phone lines so the operator simply knew where to plug in for the call. That began to change during an outbreak of the measles in Lowell, Massachusetts, in 1879. The town doctor, Moses Parker, feared that if all four Lowell operators fell ill, their substitutes would have trouble connecting people—unless every line got a number. The idea caught on.

In the 1880s telephone service quadrupled in the nation’s settled areas. Cities soon had not only a central office and phone numbers but exchanges in other parts of town, so callers now asked for Main or Central plus the subscriber’s several-digit number. Branch exchanges usually took their names from their relative geography. St. Louis had Main and Central; Baltimore, Eastern; and San Francisco, West.

As new exchanges proliferated, they usually took their names from streets or neighborhoods: thus Brooklyn’s Bensonhurst, Los Angeles’s Hollywood, and Boston’s Commonwealth. Bell devised phonetic tests to help make sure only easily understood names were chosen.

By the time dialed calling was introduced in the Bell System, in 1921, the exchange names were so ingrained that Bell Telephone kept them on. William G. Blauvelt of AT&T had divided the alphabet into groups of three letters for each of the dial’s openings in 1917. He omitted Q because of its infrequency, and the rarely used Z was relegated to the zero (operator) slot and eventually dropped as well. Because a single phone-number pulse could be transmitted when the receiver lifted or the finger wheel was jarred, no calls would be initiated until a pulse signal of at least 2 was received. Thus the number 1 got no letters attached to it.

Dialing swept the nation, but only large cities used exchange-name dialing; in small towns one still had only to dial a three- or four-digit number. For instance, in Walnut Creek, California, if your number was 1407, locally you dialed 1407. From out of town you asked for Walnut Creek 1407. Across the bay in San Francisco, if you wanted Sutter 1407, you would dial SU-1407; from afar you’d dial 211 for the long-lines operator and say, “I’d like San Francisco, please: Sutter 1407.”

When neighborhood and street names started to run out, the Bell System recommended new names. Bell of Pennsylvania looked to trees, so Pittsburgh and Philadelphia wound up in the 1930s with shared names like Locust, Poplar, and Walnut.

Seven-digit numbers became standard only after World War II. New York City had pioneered them in the early 1930s when it began inserting an “exchange-designation number” after the two-letter exchange prefix. Thus were born numbers like CAnal 6-5108. By the mid-1950s all other major cities were converted to this system, retiring such diverse combinations as Chicago’s three letters and four digits, Cleveland’s two letters and four digits, and Dallas’s one letter and four digits.

In 1961 Bell Telephone announced that it would phase out exchange-name dialing city by city. Pittsburgh and Cincinnati began conversion in 1962; Philadelphia and Seattle were the last to change, in 1978. The now-classic combination of two letters and five numbers had been a fully national standard for less than a decade.

All-number calling was introduced for several reasons. Mainly, there weren’t enough workable letter combinations. Exchanges like 571 had stayed unavailable because letters like JKL (5) and PRS (7) wouldn’t combine. All-number calling also eliminated confusingly spelled exchanges like New York’s RHinelander, prevented mix ups between similar letters and numbers like O and , and made possible direct dialing from Europe and other parts of the world. Most countries had never had letters on their dials.

The old central-office names are gone from the phone book, but they resonate in memory. They seem to stand for an era—the era of Glenn Miller’s “Pennsylvania 6-5000,” of John O’Hara’s Butterfield 8 , and of Barbara Stanwyck’s closely clutched list of phone numbers in the chilling 1948 film Sorry, Wrong Number . 335-DIET just isn’t the same.