A sound module is an electronic musical instrument without a human-playable interface such as a keyboard. Sound modules have to be operated using an externally connected device, which is often a MIDI controller, of which the most common type is the musical keyboard (although wind controllers and guitar controllers are also used). Controllers are devices that provides the human-playable interface and may or may not produce sounds of its own, or a sequencer, which is computer hardware or software designed to play electronic musical instruments. Connections between sound modules, controllers, and sequencers are generally made with MIDI (Musical Instrument Digital Interface), which is a standardized protocol designed for this purpose.