A Staff Report from the Straight Dope Science Advisory Board

Why were the star and pound symbols originally added to phone keypads?

September 23, 2015

Dear Straight Dope:

What is the history of the pound and star symbols on the phone keypad? I understand their use today for automated systems, but why were they originally added to the phone?

SDStaff Rico replies:

"A pushbutton telephone that may eventually replace the conventional [rotary] telephone dial is under development by the Bell Telephone Laboratories. Instead of twirling a dial, users will press numbered buttons to make a call - a faster process than dialing. These new sets were placed on trial last summer [summer of 1959], then taken out of service for study at the [Bell Telephone] Laboratories. Much research and testing remains to be done before the sets can be made in quantity and placed in service, but these may be the telephones of the future." - Telephone Almanac for 1960, Southern Bell Telephone and Telegraph Company/Bell Telephone System.

The original Touch-Tone phones (not simply phones with buttons) were introduced to the American public in November 1963. The first phones only had 10 buttons, the numbers 1 through 0. See a picture of this first push button phone here: http://www.thocp.net/companies/att/att_company.htm. (See "1963.") The specs, however, called for a five pointed “star” key and a “diamond” key, along with four other keys labeled A though D. The star and diamond were replaced with the asterisk and pound sign when one of the Bell System engineers decided the two extra keys on the dial should have characters drawn from the ASCII character set. Calling the asterisk "star" was easy enough, but naming the other key was more of a challenge. "Octatherp” was proposed but wisely rejected in favor of “pound.”

The engineers who designed the Touch-Tone system envisioned it in the 1940s as a multi-tone system. The familiar dual-tone system in use today (called DTMF for “Dual Tone Multi Frequency”) came about when engineers realized that a four by four grid consisting of two simultaneously sounded tones of discrete non-harmonically related frequencies would be not only more reliable but more secure. A four-by-four grid had to be used, as a three-by-three grid would have only allowed nine numbers, and you can't let six good tones go to waste! Eight tones are used in the grid, as follows:

1209 Hz 1336 Hz 1477 Hvz 1633 Hz
697 Hz 1 2 3 A
770 Hz 4 5 6 B
852 Hz 7 8 9 C
941 Hz * 0 # D

For example, the 8 key is located in the second column, which is assigned the 1336 Hz frequency, and the third row, which is assigned 852 Hz. So when 8 is pressed, both frequencies are invoked, resulting in a dual-tone beep.

As is often the cause with advances in technology, the government wanted to get involved and asked that the phone dial be adapted for special services. The result was the Autovon phone system, used only by the Department of Defense and the White House. In this system the keys A through D were used to signal a call’s priority to the government’s PBX network: Flash Override (A), Flash (B), Immediate (C), and Priority (D), with Flash Override being the highest.

Pressing one of these keys before you dialed your number gave your call priority over other conversations on the network. For example, pressing C, Immediate, before dialing would make the switch first look for any free lines. If all lines were in use, the switch would disconnect any non-priority calls, and then any Priority (D) calls. The A key, Flash Override, would kick every other call off the trunks between the origin and destination.

This system is defunct now, as advances in electronic PBX systems have made it irrelevant. The last holdout was the White House, which discontinued the system in the late 1990s.

It took a while before anyone came up with something the general public could do with the keys. Remember the Picturephone, touted as the next great thing in telephones? The pound key was to have been used as a prefix for those calls, so a Picturephone user would know when to turn the screen on.

Among the early users of the extra keys were banks. I remember using a Touch Tone phone to pay bills and check my balances through a progressive bank system in 1983. I also remember the extra charge Pacific Bell added to my bill every month to allow me to use Touch-Tone, a technology that made its switching more efficient.

After deregulation of Ma Bell in 1984, the new phone companies got wise to the extra keys and started using them to allow customers to control features on their phone systems, such as cancelling call waiting and call forwarding.

The letter keys (A, B, C, and D) are still in use today, but only for specialized applications. Technicians use them for network troubleshooting and applications, as do amateur radio operators for special controls on their systems. Those keys are available on several of my amateur radio units, and they get some use controlling several radio repeaters in my hometown.

For more information, to see an original Autovon dial, and to hear the touchtones, head on over to http://www.beatriceco.com/bti/porticus/bell/telephones-technical_dials-touchtone.html — you'll find loads of good information there, including the original press release from 1964 announcing the new technology.

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