The keys are connected to a matrix so you only need 7 microcontroller pins (3-columns and 4-rows) to scan through the pad. It's basically a sturdier version of our Membrane 3x4 Matrix Keypad and comes with 7 or 8 header pins pre-soldered on for easy plugging. Starting from the left there are three column pins, and then to the right are the four row pins. If yours has an 8th pin, its not used and you can just leave it disconnected