The MIDI protocol consists of MIDI event that carry four types of messages: 1) voice messages; 2) system common messages; 3) system realtime messages; and 4) meta messages.
MIDI voice messages are those messages that are specific to a MIDI channel and directly affect the sound produced by MIDI devices.
MIDI voice messages include the following.
| Message | MIDI device action |
| Note off | A note should be released and should stop sounding |
| Note on | A note should be played and should start sounding |
| Aftertouch / key pressure | Pressure should be applied to a note, similarly to applying pressure to electronic keyboard keys |
| Controller | A controller should be affected. A controller is a virtual slider, knob, or switch |
| Program change | A program should be assigned to a MIDI channel. A program is virtual instrument, patch, or preset |
| Channel pressure | Pressure should be applied to a MIDI channel, similarly to applying pressure to electronic keyboard keys |
| Pitch wheel / modulation wheel | A channel pitch should be changed up or down |
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