in a conventional 8 × 17 drive line / sense line keyboard switch matrix , a one - to - one correspondence between keyboard key switches and drive line / sense line pairs , together with a sequential driving of the drive lines , permits discrimination between 8 × 17 = 136 keyboard keys with 25 i / o pins . in contrast , the present invention utilizes drive line / sense line combinations to discriminate between keyboard keys with fewer total drive and sense lines , thereby reducing the number of required ic pin connections . in addition to achieving a reduction in the number of required pin connections , particular drive line / sense line contact patterns , and an associated subtraction algorithm , allow for efficient layouts of the drive lines and sense lines on respective membrane switch layers , a reduction of detection errors arising from inadvertent illegal simultaneous key presses , and discrimination of “ special ” simultaneous key press conditions .