 An individual device may be monophonic (the start of a new "note-on" MIDI command implies the termination of the previous note), or polyphonic (multiple notes may be sounding at once, until the polyphony limit of the instrument is reached, or the notes reach the end of their decay envelope, or explicit "note-off" MIDI commands are received).Receiving devices can typically be set to all four combinations of "omni off/on" versus "mono/poly" modes.A MIDI message is an instruction that controls some aspect of the receiving device.