A MIDI mode message which tells the synth to come out of a monophonic mode, if it was in one. Depending on whether Omni mode is on or off, the synth will go into MIDI channel mode 1 (in which it responds to messages on any channel) or mode 3 (the synth responds to messages only on a specific channel), and plays polyphonically. Seldom implemented in modern synths, due to the channel-modes concept having been "broken" by the advent of multitimbral synths.