It would be very helpful to many if a litle history of tone is known. Dual-tone-multi-frequency (D.T.M.F.), which is the correct generic name, was invented by or developed for the military forces radio systems. Along with tones for the numbers "zero" through "nine," there are tones for the letters "A" through "D." There are also tones for the symbols "star" and "pound."

Later, American Telephone and Telegraph Company decided to adopt this system to replace pulse dialing for their telephone switching equipment. They gave it the well-known trade-marked name of "Touch-Tone." However, as they thought that the tones for letters "A" through "D" would never be needed for their purposes, they did not include them on their telephone key pads.

Amateur radio does utilize tones "A" throuth "D" and key pads on amateur radio equipment include these tones.

There are other tone systems in use. One is called "tone-bust." I would like someone to explain if this system has been used for Americal railroad radio. It is used in amateur radio in Europe.

Another tone system in use is sub-audible tone-coded squelch or continous tone-coded squelch system (C.T.C.S.S.). The Motorola trade-marked name for this system is "Private Line" or "P.L.". It is used in many land-mobile radio systems and in amateur radio.

I invite others to expound further on these systems, as we all may benefit.