The development of the modern telephone keypad is attributed to research in the 1950s by Richard Deininger under the directorship of John Karlin at the Human Factors Engineering Department of Bell Labs. The contemporary keypad is laid out in a rectangular array of twelve push buttons arranged as four rows and three columns of keys. For military applications, a fourth, right-most column of keys was added for priority signaling in the Autovon system in the 1960s. Initially, between 1963 and 1968, the keypads for civilian subscriber service omitting the lower left and lower right keys that commonly are assigned to the star (✻) and number sign (#) signals, respectively. These keys were added to provide signals for anticipated data entry purposes in business applications, but found use in Custom Calling Services (CLASS) features installed in electronic switching systems.