One special feature of the Raspberry Pi is its [<googa>https://www.raspberrypi.org/documentation/usage/gpio/ |GPIO]</googa>, or General Purpose Input/Output connector. This is the strip of pins that runs along one side of the Pi, which acts as a connector to a number of physical inputs or outputs.
This ability to wire up custom physical inputs, switches or buttons, is part of the reason Box Of Stops was written for the Raspberry Pi. Perfect for a bit of DIY organ building.