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A telephone keypad is a keypad that appears on a “Touch Tone” telephone. It was standardised when the dual-tone multi-frequency (DTMF) system was introduced in the 1960s, and replaced the rotary dial.

The contemporary keypad is laid out in a 3×4 grid, although the original DTMF keypad had an additional column for four now-defunct menu selector keys. When used to dial a telephone number, pressing a single key will produce a pitch consisting of two simultaneous pure tone sinusoidal frequencies. The row in which the key appears determines the low frequency, and the column determines the high frequency. For example, pressing the '1' key will result in a sound composed of both a 697 and a 1209 hertz (Hz) tone.

DTMF Keypad Frequencies (with sound clips)
1 2 3 697 Hz
4 5 6 770 Hz
7 8 9 852 Hz
* :Image:Dtmf0.ogg # 941 Hz
1209 Hz 1336 Hz 1477 Hz


Note that the layout of the digits is different from that commonly appearing on calculators and numeric keypads.

The “*” is called the “star key” or “asterisk key”. “#” (while technically referred to as "octothorpe") is called the “number sign”, “pound key”, or “hash key”, depending on one's nationality or personal preference. These can be used for special functions. For example, in the UK, users can order a 7.30am alarm call from a British Telecom telephone exchange by dialling: *55*0730#.

Most of the keys also bear letters according to the following system:
A standard telephone keypad.
0 = none (in some telephones, "OPERATOR" or "OPER")
1 = none (in some older telephones, QZ)
2 = ABC
3 = DEF
4 = GHI
5 = JKL
6 = MNO
7 = P(Q)RS
8 = TUV
9 = WXY(Z)


These letters have had several auxiliary uses. Originally, they referred to exchange. In the US in the mid-20th century, numbers were seven digits long including a two-digit prefix, the latter expressed as letters rather than numbers. In the UK telephone numbering system, a similar two-letter code was added after the initial zero to form the first part of the subscriber trunk dialling code for that region - for example, Aylesburymarker was assigned 0AY6 which translated into 0296. (The majority of these original numbers have remained, particularly in the rural areas. The modern equivalent of 0AY6, namely 01296, still refers to Aylesbury.)

The letters have also been used, mainly in the U.S.marker, as a way of remembering telephone numbers easily. For example, an interior decorator might license the phone number 1-800-724-6837 but advertise it as the more memorable 1-800-PAINTER.

Sometimes advertisers will advertise a number with a mnemonic word in the number that has more letters than there are digits in the phone number. Usually, this means that you just stop dialing at 7 digits after the area code or that the numbers are ignored by the switchboard.

In recent times, the letters on the keys are needed also for entering text on mobile phones, for text messaging, entering names in the phone book, etc.; multi-tap and predictive text systems are used.

Letter mapping

When designing or selecting a new phone, publishing or using phonewords, one should be aware that there have been multiple standards for the mapping of letters to numbers on telephone keypads over the years.

The system used in the UK was different from that used in France which was different from the US etc.. The use of alphanumeric codes for exchanges was abandoned in Europe when international direct dialling was introduced in the 1960s, because (for example) dialling WHI 1212 on a French phone would get different numbers to dialling it on a British phone. At the same time letters were no longer put on the dials of new telephones.

Letters did not re-appear on phones in Europe until the introduction of mobile phones, and the layout followed the new international standard ITU E.161 / ISO 9995-8.

The keypad pictured above is mapped according to the current international standard. The ITU established an international standard (ITU E.161) in the mid-1990s, and that should be the layout used for any new devices.

Unfortunately, since many newer smartphones (such as PalmPilot and BlackBerry) have full keyboards instead of the traditional telephone keypads, the user must execute additional steps to dial a number containing convenience letters. On certain BlackBerry devices, a user can press the Alt key, followed by the desired letter, and the device will generate the appropriate DTMF tone.

Illumination

Keypads may be illuminated from inside, especially on equipment for mobile use.

See also



References

  1. Phone Key Pads
  2. E.161 : Arrangement of digits, letters and symbols on telephones and other devices that can be used for gaining access to a telephone network
  3. Blackberry Tips, PC World, October 2005.



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