The touch-tone system uses audible tones for each of the digits zero through nine. Later this was expanded by two keys labeled with an asterisk (*) and the pound or hash sign (#) to represent the 11th and 12th DTMF signals. These signals accommodate various additional services and customer-controlled calling features. The DTMF standard assigns specific frequencies to each column and row of push-buttons in the telephone keypad; the columns in the push-button pad have higher-frequency tones, and rows have lower-frequency tones in the audible range. When a button is pressed the dial generates a combination signal of the two frequencies for the selected row and column, a dual-tone signal, which is transmitted over the phone line to the telephone exchange. When announced, the DTMF technology was not immediately available on all switching systems. The circuits of subscribers requesting the feature often had to be moved from older switches that supported only pulse dialing to a newer crossbar, or later an electronic switching system, requiring the assignment of a new telephone number which was billed at a higher monthly rate. Community dial office subscribers would often find the service initially unavailable as these villages were served by a single unattended exchange, often step by step, with service from a foreign exchange impractically expensive. Rural party line service was typically based on mechanical switching equipment which could not be upgraded. While a tone-to-pulse converter could be deployed to any existing mechanical office line using 1970s technology, its speed would be limited to pulse dialing rates. The new central office switches were backward-compatible with rotary dialing. The standard layout of the keys on the touch-tone telephone was the result of research of the human-engineering department at Bell Laboratories in the 1950s under the leadership of South African-born psychologist John Elias Karlin (1918–2013), who was previously a leading proponent in the introduction of all-number-dialing in the Bell System. This research resulted in the design of the DTMF keypad that arranged the push-buttons into 12 positions in a 3-by-4 position rectangular array, and placed the 1, 2, and 3 keys in the top row for most accurate dialing. The remaining digits occupied the lower rows in sequence from left to right; the 0, however, was placed into the center of the fourth row, while omitting the lower left, and lower right positions. The DTMF keyboard layout broke with the tradition established in cash registers (and later adopted in calculators and computers) of having the lower numbers at the bottom.