Dual Tone Multi Frequency (DTMF) was first introduced in 1963 with 10 buttons in Western Electric 1500 –type telephones. DTMF was originally called TouchTone. DTMF is a more efficient means than dial pulsing for transferring telephone numbers from a subscriber’s location to the central office switching machine. DTMF is a simple two-of –eight encoding scheme where each digit is represented by the linear addition of two frequencies. DTMF is strictly for signaling between a subscriber’s location and the nearest telephone office or message switching center. DTMF is sometimes confused with another two-tone signaling system called multi frequency signaling (Mf), which is a two-of-six code designed to be used only to convey information between two electronic switching machines. Fig. shows the four-row-by-four column keypad matrix used with a DTMF keypad. As the figure shows, the keypad is comprised of 16 keys and eight frequencies. Most household telephones, however, are not equipped with the special-purpose keys located in the fourth column (i.e., the A, B, C, and D keys). Therefore, most household telephones actually use two-of-seven tone encoding scheme. The four vertical frequencies (called the low group frequencies) are 697 Hz, 852 Hz, and 941 Hz, and the four horizontal frequencies (called the high group frequencies) are 1209 Hz, 1336 Hz,1477 Hz, and 1633 Hz. The frequency tolerance of the oscillators is + .5%. As shown in Figure, the digits 2 through 9 can also be used to represent 24 of the 26 letters (Q and Z are omitted). The letters were originally used to identify one local telephone exchange from another.