A MIDI message that tells the receiver to begin sounding a note. The message contains the note number of the note, and a velocity value that encodes how rapidly or with how much force the key was pressed. The receiver is supposed to use the velocity value to scale the amplitude and/or attack time of the note sounded; it may also use the value to perform other functions, such velocity switching on a sampler. The note on message does not specify a duration for how long the note is to be held; it is indefinite until a corresponding note off message containing the same note number (or an all notes off) is received.
Because MIDI limits data bytes to the range 0-127, and the note number in the message is a single byte, in the equal tempered scale (12 notes per octave), the note number range covers about 10-1/2 octaves. This is more than the range of any musical instrument ever built. By convention, note #60 is middle C, which has a frequency of about 261 Hz. Extending the note number range down to note #0 (a C five octaves below middle C) yields a frequency value of about 8 Hz, and extending up to note 127 (a G that is 5 octaves and 7 half steps above middle C), produces a frequency value of about 12 KHz. If the note numbers are mapped to alternate scales which have more than 12 notes per octave, the range is obviously reduced; this is one of the criticisms of MIDI's note numbering method.
Per the MIDI standard, a note on message with a velocity of zero is equivalent to a note off. Some older synths, particularly some Yamaha synths (e.g., early production DX7s) did not implement this properly.
A problematic aspect of this method of specifying notes to play is what the receiver should do when it receives a note on message containing the number of a note that is already sounding. What the receiver should do in this situation is left unspecified by the MIDI standard. Some synths will retrigger the note, while some other synths will simply ignore the message. Some polyphonic synths will actually allocate a second voice to that note number, and it will begin sounding in unison with the voice that is already sounding that note (a situation called "double voicing"). In this case, the synth may require two note-off messages for that note number, in order to release both voices. For this reason, a design principle for sequencers is that every note on message must be paired with a note off, even if the note off might be redundant.